RUSSIAN DRONE INCURSIONS: NATO’S EASTERN FLANK BELOW ARTICLE 5

In November 2025, I described Russia’s drone incursion into Poland as a “low-end stress test” for the North Atlantic Treaty Organisation (hereinafter: the NATO): a challenge sufficiently serious to test air defence, alliance cohesion and escalation control, yet limited enough to remain below the conventional threshold of collective defence. Less than a year later, the more consequential development is no longer the existence of such tests. It is their repetition, geographical diffusion and gradual incorporation into the ordinary security environment of NATO’s eastern flank. What initially appeared as a problem of episodic spillover from the war in Ukraine increasingly resembles a persistent condition that allies must detect, interpret and manage on a recurring basis.

The change is visible from the Black Sea to the Baltic. Romania, after recording dozens of unauthorised drone entries since Russia’s full-scale invasion of Ukraine, crossed an important operational threshold in July 2026 when its F-16 fighter jets shot down Russian drones violating national airspace for the first time; three were destroyed within three days. In August, an F-18 fighter jet operating under NATO air policing destroyed another drone after it entered Romanian territory from Moldova. The pattern continued in September. Two Russian drones entered Moldovan airspace on 9 September, one crashing and causing a fire while another continued into Romania. Poland simultaneously warned that Russia could increasingly target border crossings linking Ukraine with the European Union, and on 13 September a Russian drone struck a passenger train close to the Polish border. The following day, Polish forces reported finding what appeared to be a Russian military drone near the Baltic coast. Individually, these incidents differ in origin, intent, military relevance and legal significance. Collectively, however, they indicate that exposure to the Russia-Ukraine war is no longer confined to occasional debris or navigational error along a single frontier. This distinction matters because not every violation should be interpreted as deliberate Russian escalation. A drone diverted by electronic warfare, a platform that crosses a border during an attack on Ukrainian infrastructure and an aircraft deliberately sent to probe NATO reaction times are strategically different events even when their radar signatures initially appear similar. Attribution of intent, therefore, requires caution. Yet uncertainty about individual incidents does not eliminate the significance of the aggregate pattern. Repeated violations oblige frontline states to scramble aircraft, activate air-defence systems, interrupt civilian aviation, deploy surveillance assets and maintain forces at higher readiness. Even an action too limited to constitute an armed attack can therefore impose military, economic and political costs when it occurs often enough.

NATO has already begun adapting to this environment. Eastern Sentry, launched following the 2025 incursions into Polish and Estonian airspace, connects allied air, surveillance and defensive capabilities across an eastern flank stretching from the Black Sea towards the High North. NATO officials have also acknowledged the need for cheaper and more scalable counter-drone systems, partly because reliance on conventional interceptors creates an unfavourable cost exchange against inexpensive unmanned platforms. The Alliance is consequently becoming better equipped to detect and defeat individual incursions. That operational adaptation, however, does not by itself resolve the strategic problem created by their recurrence.

The central challenge is therefore not simply whether NATO can shoot down another drone. Nor is it whether every ambiguous incident should move the Alliance closer to Article 5. Framing the problem in either way creates a false choice between passivity and escalation. The more difficult task is threshold management: constructing a sufficiently credible, economical and predictable system of responses to coercive or risk-producing activity that remains below the conventional threshold of collective defence. NATO must be able to deny repeated violations operational value without allowing every incursion to become a strategic crisis; impose costs without creating automatic escalation; and distinguish genuine accidents from patterns of behaviour that progressively normalise interference with allied territory.

This article argues that the eastern flank is moving from an era of discrete incursions towards a persistent sub-threshold security environment. The strategic danger lies less in any single drone crossing a border than in the cumulative normalisation of low-cost actions that force NATO repeatedly to spend resources, absorb disruption and make politically consequential decisions under conditions of uncertain intent. The appropriate response is not to lower the threshold of Article 5. It is to build a stronger architecture beneath it. Deterrence on NATO’s eastern flank will increasingly depend on whether the Alliance can make sub-threshold coercion predictable to answer, expensive to repeat and strategically unrewarding — without converting every provocation into a step towards direct war with Russia.

From Spillover To Pattern

Airspace violations along NATO’s eastern flank are not new. Since Russia’s full-scale invasion of Ukraine, missiles, drones and debris have repeatedly approached or crossed the borders of neighbouring states. What has changed is not simply their number. The relevant shift is the combination of frequency, geographical dispersion and operational consequence. Incidents once discussed primarily as spillover from attacks on western Ukraine now form part of a wider security geography extending from the Black Sea through Poland and the Baltic states towards the Alliance’s northern flank. NATO itself increasingly describes Russian behaviour in these terms: Eastern Sentry was launched after the unusually extensive airspace violations of September 2025, and the Alliance now characterises the broader trend as an increase in reckless and escalatory actions along its eastern borders.

Yet identifying a pattern does not mean assuming a common intention behind every incident. For analytical purposes, at least three different mechanisms should be distinguished.

The first is accidental spillover. Modern drone warfare occurs in a contested electromagnetic environment in which navigation can fail, electronic warfare can alter flight paths and damaged platforms can travel considerable distances before crashing. A Russian drone entering NATO airspace may therefore do so without having been programmed to attack or reconnoitre NATO territory. Similar uncertainty applies to unidentified objects detected by increasingly sensitive surveillance systems. In September 2026, for example, Lithuania temporarily closed Vilnius airport and NATO launched a fighter in response to a suspected drone intrusion; visual inspection subsequently established that the radar contacts were a flock of birds. The episode was harmless, but it illustrates a broader problem: heightened readiness increases the likelihood that ambiguous signals will require immediate operational responses before their nature can be established.

The second category is better understood as reckless operational externality. Here, an incursion may not constitute a deliberate attack on NATO, yet it results directly from Russian military operations conducted close enough to allied territory, and with sufficiently limited regard for cross-border consequences, that violations become structurally foreseeable. The distinction between accident and recklessness matters. If repeated strikes against Ukrainian targets situated close to Moldova, Romania or Poland routinely create risks beyond Ukraine’s borders, the resulting incursions cannot indefinitely be treated as isolated navigational anomalies. NATO Secretary General Mark Rutte has made a similar distinction, noting that the precise intent behind individual violations may remain uncertain while the underlying behaviour can nevertheless be classified as reckless. Recent events around Moldova and Romania demonstrate the practical implications. On 9 September, Moldovan authorities reported that two Russian drones entered the country’s airspace during attacks on neighbouring Ukraine. One crashed and caused a fire, while another continued into Romania. Moldovan airspace was temporarily closed, delaying civilian flights and the departure of Ukrainian President Volodymyr Zelenskyy. No attack on Moldova or Romania had to occur for the operation to produce consequences inside both countries. The military action remained centred on Ukraine, but its security and economic externalities crossed international borders.

The third and most consequential possibility is deliberate probing: the intentional use of limited incursions or other low-level military activity to collect information about NATO’s detection capabilities, reaction times, rules of engagement, political coordination or tolerance for repeated violations. Establishing such intent in any particular case is difficult and should not be inferred merely from the nationality of a drone or the location in which it is found. The suspected Russian military drone discovered near Poland’s Baltic coast on 14 September, for instance, raises legitimate questions about origin and purpose, but publicly available information does not yet establish why it was there. Analytical caution is therefore essential.

The problem for NATO is that these categories are strategically distinct but operationally intertwined. Air-defence personnel cannot wait for definitive attribution before deciding whether an unidentified platform approaching protected airspace requires interception. A fighter aircraft may therefore have to scramble against an object that later proves harmless; an expensive interceptor may be used against an inexpensive drone whose border crossing was accidental; or an apparently minor intrusion may turn out to have been designed precisely to observe how the Alliance responds. The defender bears the informational burden because the nature of the incident often becomes clear only after the operational decision has already been made. This asymmetry becomes more important as incidents accumulate. NATO’s response since September 2025 reflects recognition that the problem can no longer be managed exclusively as a series of nationally isolated border violations. Eastern Sentry combines fighter aircraft, surveillance platforms, air-defence systems, naval assets and other capabilities along the entire eastern flank, while NATO Air Policing and Airborne Warning and Control System (hereinafter: the AWACS) missions provide continuous monitoring of the airspace surrounding the Alliance. NATO officials have explicitly described the activity as extending from the Black Sea to the High North and as a mechanism for identifying gaps in allied detection and counter-drone capabilities.

The analytical transition is therefore from incident attribution to pattern recognition. Whether every drone was deliberately sent across a NATO border is ultimately less important than whether repeated incursions create a recurring requirement for NATO to detect, classify, intercept and politically interpret ambiguous threats. Accidental spillover, reckless externality and deliberate probing should not be conflated. But neither should uncertainty over intent obscure their cumulative effect. When different types of intrusion repeatedly generate the same defensive burden, what begins as spillover gradually becomes part of the security environment itself.

The Article 5 Trap

NATO’s difficulty in responding to repeated low-level incursions is partly a consequence of the strength of its principal deterrent. Article 5 of the North Atlantic Treaty is designed around an armed attack against an Ally. It does not establish an automatic military response: each Ally is required to assist the attacked state by taking such action as it considers necessary, which may include the use of armed force. Nor does the Treaty prescribe a mechanical procedure through which Article 5 is triggered. The distinction nevertheless remains fundamental. Collective defence carries a political and strategic weight deliberately greater than the routine management of border incidents, airspace violations or other security threats below the level of armed attack. This creates what might be called the Article 5 trap. The stronger and more credible NATO’s commitment to respond collectively to an armed attack becomes, the greater the incentive for hostile activity to remain beneath that threshold. An adversary does not necessarily need to challenge collective defence directly if it can generate disruption, impose defensive expenditure, collect information or create political uncertainty through actions sufficiently limited to make escalation appear disproportionate. The resulting strategic space is not one of peace in the conventional sense, but neither is it easily governed by the mechanisms designed for war.

NATO is not institutionally defenceless in this space. Article 4 provides for consultations whenever an Ally considers its territorial integrity, political independence or security threatened. Poland invoked it following the large-scale Russian drone violation of its airspace on 10 September 2025, and Estonia did so after three Russian MiG-31 aircraft entered Estonian airspace later that month. These consultations were followed by military adaptation, most notably the launch of Eastern Sentry. Yet Article 4 is fundamentally a consultation mechanism rather than a predetermined ladder of consequences. It enables Allies to recognise a threat collectively; it does not specify what particular cost should follow a second, fifth or twentieth occurrence of similar behaviour.

The resulting dilemma can be expressed as a tension between normalisation and escalation. Persistent underreaction risks making repeated violations part of the accepted operating environment. If every incursion produces surveillance, diplomatic condemnation and temporary reinforcement but no additional consequence for the actor responsible, the cumulative political signal may become weaker even while NATO’s tactical response remains competent. What was initially exceptional can gradually become routine.

Overreaction creates the opposite danger. Not every airspace violation constitutes an armed attack, and not every Russian-origin platform entering allied territory demonstrates an intention to attack NATO. Treating ambiguous or limited incidents as automatic steps towards collective defence could compress the political space available for investigation, proportionality and escalation control. It could also allow relatively inexpensive actions to generate strategically costly allied responses. The central problem is therefore not simply how to demonstrate resolve, but how to do so proportionately enough to preserve escalation control and consistently enough to prevent normalisation.

NATO already recognises that the boundary between an isolated incident and an armed attack cannot always be understood through a single event. The Alliance has repeatedly stated that hybrid operations could, depending on their effects, reach the level of an armed attack and lead the North Atlantic Council to invoke Article 5. Its cyber policy goes further by explicitly recognising that the cumulative effects of multiple malicious activities may, in certain circumstances, reach that threshold. This principle is important because it acknowledges that strategic harm can be produced through accumulation rather than through one spectacular act.

Yet recognising cumulative harm does not solve the more common problem: what should NATO do while cumulative activity remains serious enough to matter but insufficient to constitute an armed attack? This is precisely the space in which repeated drone incursions, electronic interference, sabotage risks and pressure on logistical infrastructure become strategically significant. The choice cannot be reduced to either invoking Article 5 or accepting the activity. NATO requires credible instruments between those poles.

The imbalance is particularly favourable to the initiator of sub-threshold activity. A limited action can force the defender to answer several questions simultaneously: Was it deliberate? Who authorised it? Does it form part of a broader campaign? Is interception sufficient? Should attribution be public? Should political, economic or military costs follow? The actor creating the ambiguity therefore imposes not only operational costs but also a decision-making burden. Every incident forces NATO to repeat an attribution, proportionality and escalation calculation that the initiator may have designed precisely to complicate. This is why Article 5 should remain a high threshold rather than become the default answer to low-level coercion. Its deterrent value partly derives from the seriousness of the commitment it represents. Lowering that threshold in response to every ambiguous violation could ultimately weaken rather than strengthen its credibility. The missing element is instead a more developed architecture beneath it: one capable of ensuring that activities which do not justify collective defence nevertheless produce consequences that are sufficiently predictable to affect an adversary’s calculations.

The Article 5 trap is therefore not a flaw in the Washington Treaty. It is a response gap between consultation and collective defence. NATO has a well-established mechanism for discussing threats through Article 4 and an exceptionally powerful mechanism for responding to armed attack through Article 5. The emerging challenge on the eastern flank lies between them. Closing that gap requires the Alliance to move beyond asking when an incident becomes serious enough for Article 5 and towards determining how repeated activity should be deterred before it ever reaches that point.

Eastern Sentry: Adaptation Without Resolution

NATO has not remained static in the face of the changing security environment on its eastern flank. The most visible institutional response has been Eastern Sentry, launched in September 2025 after the unprecedented concentration of Russian drone and aircraft violations of Allied airspace. Unlike a temporary reinforcement confined to one exposed state, Eastern Sentry was designed as a flexible, multi-domain activity covering the Alliance’s eastern flank as a whole. Allies have contributed additional fighter aircraft, helicopters, surveillance and transport platforms, air-defence systems and naval assets, while NATO has sought to coordinate these capabilities more closely through its existing command structure. The significance of the initiative is therefore not merely quantitative. Eastern Sentry represents an attempt to treat dispersed incidents as a theatre-wide security problem rather than as separate national emergencies.

This shift has produced tangible operational advantages. First, it improves situational awareness. NATO Air Policing already provides continuous surveillance of Allied airspace, while AWACS aircraft and national sensors contribute a broader picture of activity beyond individual borders. Eastern Sentry reinforces this architecture by bringing additional surveillance platforms and national capabilities into a more integrated operational framework. NATO’s RQ-4D Phoenix remotely piloted aircraft, for example, supported Eastern Sentry missions from Norway and Finland in 2026, providing persistent intelligence, surveillance and reconnaissance along parts of the northern and northeastern flank. The aim is not simply to see more objects, but to connect information more rapidly across national boundaries and command levels.

Second, the initiative increases response flexibility. An airspace violation near Romania does not have to be understood exclusively as a Romanian problem, just as an intrusion near Poland need not depend solely on Polish assets. The military logic of Eastern Sentry is to make a wider pool of Allied capabilities available within a common defensive posture. NATO demonstrated this principle in March 2026, when Allied aircraft operated alongside Romanian F-16s during heightened activity on the Black Sea flank. The broader Integrated Air and Missile Defence framework similarly links Air Policing, ground-based air defence, surveillance and other capabilities that can be employed according to the nature of the threat.

Third, Eastern Sentry has accelerated technological adaptation. The drone problem has exposed a mismatch between parts of NATO’s traditional air-defence architecture and the economics of mass-produced unmanned systems. High-performance aircraft and sophisticated surface-to-air missiles remain indispensable against advanced threats, but they are an inefficient default response to every inexpensive drone. NATO has consequently begun experimenting more systematically with counter-drone technologies, autonomous systems, new communications architectures and other emerging capabilities on the eastern flank. During exercises in Slovakia in June 2026, Allied forces tested drones, counter-drone systems, uncrewed ground vehicles and next-generation communications as part of a broader Eastern Flank Deterrence Initiative.

NATO’s leadership has also acknowledged the underlying cost problem explicitly. Responding in June 2026 to concerns that drone intrusions in Latvia had become increasingly frequent, Secretary General Mark Rutte argued that the Alliance could not sustainably rely on expensive interceptors against comparatively cheap drones and would need counter-drone systems with a more favourable cost relationship. This is an important recognition. Deterrence depends not only on whether NATO can destroy an incoming platform, but also on whether it can do so repeatedly without allowing an adversary to impose disproportionate costs through inexpensive means.

These adaptations should not be understated. Eastern Sentry makes NATO more capable of detecting, tracking and intercepting threats; improves the sharing and coordination of assets across national boundaries; strengthens visible Allied solidarity; and creates an institutional setting in which lessons from Ukraine can be incorporated more quickly into NATO defence planning. It also reduces the risk that an exposed eastern Ally must interpret and manage an ambiguous incident largely on its own. In this respect, the activity directly addresses several weaknesses exposed by the incursions of 2025. Yet operational adaptation is not equivalent to strategic resolution.

Eastern Sentry primarily strengthens NATO’s ability to manage the consequences of an intrusion. It does not automatically determine how responsibility should be attributed when intent is ambiguous, what additional political or military cost should follow repeated violations, or at what point a succession of individually limited acts should be treated as a coordinated coercive campaign. Better radar coverage can establish that an object crossed a border. It cannot by itself establish whether that crossing was accidental, reckless or deliberate. A fighter can intercept a drone. The interception does not necessarily alter the incentive structure that produced the incident.

The same distinction applies to reassurance. Deploying additional aircraft and air-defence systems signals Alliance cohesion and reduces vulnerability, but reassurance to Allies and deterrence of an adversary are related rather than identical functions. NATO can demonstrate that it is prepared to defend Romanian, Polish or Baltic airspace while an adversary simultaneously concludes that repeated low-level incursions remain tolerable because each one produces a largely defensive response. In such circumstances, tactical success may coexist with strategic persistence: NATO successfully defeats the individual intrusion, yet the pattern continues. This is the central limitation of adaptation without a corresponding response architecture. Eastern Sentry improves the Alliance’s ability to answer the question, What should we do when another object enters NATO airspace? It does not fully answer the more difficult question, What should happen to the actor responsible when similar incidents continue to occur? Detection, attribution and deterrence belong to the same security problem, but they are not the same function.

Eastern Sentry should therefore be understood as a necessary first layer rather than a complete solution. It reduces vulnerability and increases NATO’s operational options. What it cannot do on its own is transform repeated sub-threshold activity from a manageable nuisance into an unattractive strategic choice for the actor generating it. That requires a second layer: a response framework capable of linking recurrence to progressively more predictable consequences. Without such a framework, NATO may become increasingly efficient at managing the symptoms of the new security environment while leaving the incentives that sustain it largely unchanged.

The Economics Of Low-End Deterrence

The challenge posed by repeated drone incursions is not only military. It is also economic. Sub-threshold coercion is particularly attractive when a relatively inexpensive action can force a substantially more expensive defensive response. This does not mean that every Russian drone entering or approaching NATO airspace is deliberately intended to exhaust Allied resources. It does mean that, regardless of intent in an individual case, repeated low-cost intrusions can generate a structurally unfavourable cost-exchange relationship for the defender.

The most visible version of this problem is the mismatch between inexpensive unmanned aircraft and sophisticated interception systems. NATO Secretary General Mark Rutte acknowledged the problem directly in June 2026, arguing that the Alliance could not sustainably continue using very costly interceptors to destroy comparatively cheap drones and would therefore need counter-drone technologies operating at a similar cost level to the systems they were designed to defeat. His formulation captures an important principle of modern air defence: interception capability is not sufficient if the economics of interception cannot be sustained at scale. A system capable of destroying ninety-nine out of one hundred drones may still represent an unfavourable strategic exchange if every interception consumes resources many times more valuable than the incoming threat.

Yet the economics of low-end deterrence extend far beyond the price of the munition used to destroy a drone. An unidentified object approaching Allied airspace can require radar tracking, command-and-control attention, intelligence assessment and, in some cases, the scrambling of fighter aircraft. Air-defence units may have to increase readiness, civilian aviation authorities may alter traffic, airports may temporarily suspend operations and emergency services may be placed on alert. None of these measures requires an attack to succeed. The defensive cost is generated by the possibility that the object might represent a genuine threat.

Recent incidents illustrate this wider burden. On 13 September 2026, Lithuania temporarily closed Vilnius airport after radar contacts raised concerns about a possible drone intrusion, while NATO dispatched a fighter aircraft to investigate. The objects were subsequently identified as birds and the airport reopened after thirty-eight minutes. From a security perspective, the system worked: an ambiguous signal was detected, investigated and safely resolved. From an economic perspective, however, the episode demonstrates that uncertainty itself carries a price. An actor seeking to impose disruption does not necessarily need to penetrate an air-defence network successfully if the defender must react seriously to every credible warning.

The same logic is visible further south. When Russian drones entered Moldovan airspace on 9 September, Moldova temporarily closed its airspace, delaying arriving and departing civilian flights as well as the onward travel of Ukrainian President Volodymyr Zelenskyy. One of the drones crashed and caused a fire; another continued towards Romania. Again, the relevant economic effect did not depend on an intended strike against civilian aviation. The presence of an uncertain airborne threat was sufficient to interrupt normal activity.

Border infrastructure creates another channel through which limited military activity can generate disproportionate costs. Poland has reinforced crossings with Ukraine after recent Russian strikes close to key transport nodes, constructing protective positions and strengthening observation and guard facilities at Dorohusk, Medyka and Korczowa. Such measures are rational and increasingly necessary. But they also illustrate how the defensive burden expands geographically: an attack conducted primarily against Ukraine can require additional expenditure, personnel and infrastructure protection inside NATO territory. The cost imposed on the Alliance is therefore not limited to the object that crosses its border. It includes the defensive adaptation required because similar events may recur. This distinction leads to a broader understanding of the economics of imposed readiness. Military readiness is valuable precisely because it reduces the time required to respond to a threat. But maintaining higher readiness for prolonged periods consumes resources. Aircraft accumulate flight hours; crews require rotation and training; radar and surveillance networks operate continuously; air-defence units must remain deployable; and munitions, spare parts and maintenance capacity must be available in sufficient quantities. A short-lived crisis can absorb these costs as an exceptional requirement. A persistent sub-threshold environment turns them into structural expenditure.

NATO’s current investment trajectory demonstrates the scale of this adjustment. At the July 2026 NATO Summit Defence Industry Forum in Ankara, Allies announced plans to invest more than $40 billion in counter-drone capabilities over the following five years, alongside expanded training for drone operators and the creation of a NATO marketplace intended to accelerate procurement of tested and interoperable counter-drone systems. The investment is strategically justified, but its magnitude also reveals an important feature of the contest: relatively simple unmanned systems are forcing advanced military alliances to reorganise procurement, training and industrial capacity around the problem of defeating them economically as well as operationally. This is why the relevant metric should not be the price of a drone compared with the price of a missile. The more useful measure is the total defensive cost generated per unit of hostile or ambiguous activity. That calculation includes interception where necessary, but also surveillance, sortie generation, airport disruption, infrastructure protection, command attention and the opportunity cost of forces that could otherwise be employed elsewhere. It also includes adaptation costs: once a recurring vulnerability is identified, states must invest to prevent its exploitation in the future.

The strategic asymmetry follows directly. Russia does not need to win an air engagement to impose costs on NATO. It does not even necessarily need a drone to reach its intended destination. If an inexpensive platform forces several Allied systems to activate, changes civilian behaviour, consumes military readiness and contributes to long-term defensive investment, part of its strategic effect has already been generated. Repetition can magnify this effect because each incident must initially be treated according to the potential threat it presents rather than the harmlessness that might become apparent afterwards. This does not imply that NATO should respond less frequently or tolerate unidentified aircraft in order to save money. Such a policy would create exactly the vulnerability that deterrence is intended to prevent. The objective must instead be to reverse the cost-exchange relationship. Low-cost threats require layered low-cost responses wherever possible: electronic warfare, inexpensive ground-based interceptors, directed-energy or gun-based solutions where technically appropriate, autonomous detection and classification, and cheaper interceptor drones before high-end fighter aircraft or scarce surface-to-air missiles are employed. NATO’s growing emphasis on counter-drone experimentation and procurement reflects precisely this requirement. In June 2026 alone, hundreds of military and industry specialists tested dozens of systems and software applications in NATO’s principal counter-drone interoperability exercise.

But technological efficiency addresses only half of the problem. A cheaper interception system reduces the operational cost of defending NATO airspace; it does not create a cost for the actor whose behaviour repeatedly generates the defensive requirement. Sustainable low-end deterrence, therefore, requires two complementary forms of cost management: reducing the price NATO pays for each incident and increasing the expected price of deliberately repeated coercion. The first is primarily a technological and procurement challenge. The second is political and strategic. This distinction is crucial. If NATO merely becomes cheaper and faster at intercepting drones, it can manage the new environment more efficiently, but may leave the adversary’s incentives unchanged. If it relies only on punishment, however, it risks escalating ambiguous incidents whose intent may not be established. The more durable solution is consequently a combination of economical defence and calibrated consequences. NATO must make individual incursions inexpensive to defeat while making deliberate repetition progressively more costly to sustain.

The economics of low-end deterrence, therefore, lead back to the central problem of threshold management. The strategic contest is not simply over who can deploy the more advanced technology. It is over who can force the other side to spend more political, military and economic resources to maintain the same level of security. NATO’s task is to ensure that defending the eastern flank does not become a permanently unfavourable exchange in which inexpensive ambiguity repeatedly purchases expensive Allied attention.

From Threshold Deterrence To Threshold Management

The preceding sections point to a problem that cannot be solved simply by strengthening air defence. NATO can improve detection, acquire cheaper interceptors and deploy additional forces along its eastern flank, yet still confront the same strategic dilemma if repeated sub-threshold activity produces no predictable change in the consequences faced by the actor responsible. A sustainable response, therefore, requires NATO to complement threshold deterrence — the threat of a powerful response once a sufficiently serious line has been crossed — with threshold management: a structured process for dealing with hostile or risk-producing activity before it reaches the level of collective defence.

Threshold management should not be understood as replacing the ambiguity surrounding Article 5 with a rigid set of automatic triggers. Such an approach would be strategically undesirable and institutionally unrealistic. NATO deliberately assesses whether an incident constitutes an armed attack on a case-by-case basis, and Allies retain political control over the nature of their response. Strategic ambiguity also has deterrent value because an adversary cannot know with certainty where every possible form of aggression will meet the collective-defence threshold. The objective should therefore be to preserve ambiguity at the upper end of escalation while creating greater predictability below it.

NATO already possesses many of the institutional components required for such an approach. Air Policing and Integrated Air and Missile Defence provide continuous surveillance and interception capabilities; Eastern Sentry adds theatre-wide reinforcement and coordination; Article 4 permits political consultation when an Ally considers its security threatened; intelligence-sharing supports attribution; and NATO’s counter-hybrid framework allows the Alliance to assist an Ally at different stages of a hybrid campaign. NATO has also long recognised that hybrid activity can, in sufficiently serious circumstances, result in a North Atlantic Council decision to invoke Article 5. The missing element is therefore not an entirely new institution. It is a more explicit connection between recurrence, confidence of attribution and progressively consequential responses.

A practical threshold-management architecture could be organised around five levels.

The first level would cover isolated or genuinely ambiguous incidents. These include objects whose origin, intent or even nature cannot initially be established. The appropriate response is primarily defensive: detection, classification, interception where necessary, technical investigation and rapid intelligence-sharing. Public attribution should remain cautious until evidence permits greater confidence. The purpose at this stage is not punishment but denial: the object should be prevented from producing military advantage while the Alliance avoids transforming uncertainty into unnecessary escalation.

The second level would apply to repeated territorial violations for which the source can be identified with reasonable confidence, even if hostile intent remains uncertain. At this point, recurrence itself becomes strategically relevant. NATO should move from isolated incident management towards systematic documentation and collective attribution. Interception procedures can become more automatic within existing rules of engagement, intelligence from several Allies should be pooled, and the North Atlantic Council should receive regularised assessments of the cumulative pattern. The political signal should also change. Rather than treating each violation as an entirely new event, NATO should state publicly when it considers incidents part of an established pattern of reckless behaviour.

The third level would concern repeated activity for which there is substantial evidence of deliberate probing or coercive intent. Here, denial alone is insufficient because the problem is no longer merely the physical object entering Allied airspace but the incentive to repeat the behaviour. NATO and individual Allies should therefore have a menu of pre-agreed, proportionate countermeasures that can be applied without requiring a fresh strategic debate after every incident. These need not be symmetrical. Measures could include strengthened forward deployments, intensified surveillance of the responsible actor’s military activity, coordinated diplomatic expulsions, targeted economic restrictions, additional support to Ukraine, or other reversible military and political steps. The purpose would be to establish a simple expectation: deliberate repetition produces an increasing price.

The fourth level would apply when sub-threshold activity begins to threaten critical infrastructure, major transport links, energy networks, border logistics or civilian safety in a sustained way. At this stage, the significance of an incident should be assessed not only by the platform used but by its effects. A low-cost drone that repeatedly disrupts a major airport, a coordinated campaign against rail links supplying Ukraine, or combined physical and cyber pressure on energy infrastructure may have greater strategic consequences than the sophistication of the weapon itself suggests. Article 4 consultations should become a normal rather than exceptional instrument at this level, and NATO should be prepared to combine military reinforcement with economic, cyber, intelligence and diplomatic responses. The Alliance’s own doctrine already recognises that hybrid attacks can have effects sufficiently serious to raise collective-defence questions.

The fifth level would remain the domain of potential collective defence. An armed attack producing significant destruction, casualties or other effects judged by Allies to cross the Article 5 threshold would require consideration by the North Atlantic Council under the existing Treaty framework. Nothing in a threshold-management architecture should predetermine that decision. Its purpose is precisely the opposite: to create credible options before this level is reached, thereby reducing the likelihood that NATO faces an artificial choice between tolerating repeated coercion and moving directly towards collective defence.

Such a system would work best if escalation across the levels were guided by several variables rather than by a single numerical trigger. Frequency matters because repeated behaviour is different from an isolated accident. Attribution confidence matters because punitive measures require a stronger evidential foundation than interception. Intent indicators matter because reconnaissance, navigational error and deliberate attack carry different implications. Effects matter because a technically simple action may generate severe disruption. Finally, cross-domain coordination matters: a drone incursion occurring simultaneously with cyber disruption, GPS interference or sabotage attempts should be assessed differently from an otherwise similar incident occurring in isolation. This approach offers an important advantage over a rigid red-line strategy. It preserves NATO’s ability to adapt its response to circumstances while reducing the adversary’s ability to exploit the assumption that every sub-Article 5 incident will begin a new political debate from zero. Predictability need not mean announcing in advance that “incident X will automatically produce response Y”. It means establishing that patterns have consequences, that recurrence changes the Alliance’s assessment and that repeated behaviour progressively narrows the space for cost-free coercion.

The distinction is especially important because deterrence operates through expectations. An adversary evaluating a limited incursion is not only asking whether NATO can intercept the platform. It is also asking what will happen afterwards. If the answer depends almost entirely on an improvised political process each time, ambiguity can favour the initiator. If repeated behaviour predictably leads to greater surveillance, stronger deployments, economic measures, intensified support to the adversary’s opponent or other cumulative costs, the calculation changes even though Article 5 remains untouched.

Threshold management would also help solve the cost problem identified earlier. NATO should seek to make defence cheaper at lower levels and consequences progressively more expensive at higher ones. An isolated drone should ideally be defeated by an economical counter-drone system rather than by a scarce high-end interceptor. A deliberate campaign of repeated incursions, however, should begin to generate costs outside the immediate air-defence engagement. The objective is to prevent an adversary from choosing both sides of the exchange: imposing expensive defensive requirements while itself remaining insulated from consequences. There are, however, two important safeguards. First, escalation must remain reversible wherever possible below the armed-attack threshold. Measures that can be intensified, suspended or withdrawn give political leaders room to signal resolve without creating an irreversible path towards conflict. Second, the evidentiary standard should rise with the severity of the response. NATO can intercept an unidentified aircraft because of the immediate risk it poses without proving hostile intent; imposing significant punitive costs requires stronger attribution. Operational caution and political proportionality should therefore increase together rather than be treated as competing principles.

This approach is consistent with NATO’s broader adaptation to an environment increasingly defined by hybrid threats and recurrent shocks. The 2026 Ankara Summit reaffirmed that the Alliance must continue adapting across conventional, cyber, space, intelligence and uncrewed capabilities, while earlier NATO doctrine had already established preparedness, deterrence and defence against hybrid activity as collective tasks. Threshold management would connect these elements more systematically. It would transform a collection of defensive instruments into a graduated logic of response.

The aim is therefore not to eliminate the grey area between peace and collective defence; no institutional framework can remove uncertainty from military competition. The aim is to make that area harder to exploit. NATO should preserve strategic ambiguity over the precise upper threshold at which collective defence becomes necessary while reducing ambiguity over whether repeated coercion below that threshold will carry consequences. In practical terms, the principle is straightforward: uncertainty over when Article 5 applies should not become certainty that everything below it is tolerable.

Four Pathways And What To Watch

Threshold management becomes useful only if NATO can distinguish between different trajectories of sub-threshold activity. The existence of repeated incursions does not demonstrate that every incident forms part of a centrally coordinated Russian escalation strategy. Nor does the absence of an armed attack mean that the security environment is stable. The more useful question is therefore not whether escalation is occurring in the abstract, but what kind of pattern is emerging and what evidence would distinguish one trajectory from another.

Four broad pathways are plausible: managed spillover, calibrated probing, infrastructure coercion and threshold crisis. They are not mutually exclusive, nor must one inevitably develop into the next. Elements of several pathways may coexist. Their analytical value lies in identifying the indicators that should alter NATO’s assessment as events accumulate.

Managed Spillover

The least escalatory pathway is one in which most incursions remain unintended consequences of a continuing high-intensity war close to NATO territory. Russian forces continue attacking Ukrainian targets near Poland, Romania and Moldova; electronic warfare, navigation failures or damaged drones occasionally carry platforms across borders; NATO intercepts them when necessary; and no convincing evidence emerges that Russia is systematically directing such incidents against Allied territory. This would still constitute a serious security problem. Spillover can kill civilians, disrupt airports, damage infrastructure and produce miscalculation even without hostile intent towards NATO. The September 2026 drone incursions into Moldova and Romania demonstrate precisely this danger: Russian operations directed against Ukraine produced airspace closures and civilian disruption beyond the country being attacked.

The principal indicators of managed spillover would be geographical proximity to ongoing strikes in Ukraine, irregular timing, inconsistent flight behaviour and an absence of repeated concentration around specific NATO military or infrastructure targets. Incidents would correlate strongly with the intensity and geography of Russian strikes on neighbouring Ukrainian regions rather than with identifiable NATO exercises, deployments or political decisions.

If this pathway predominates, NATO’s priority should remain denial and resilience rather than punishment. Better sensors, inexpensive counter-drone systems, rapid information exchange and deconfliction mechanisms would reduce the risks without requiring every violation to become a political confrontation. The relevant measure of success would be the Alliance’s ability to make spillover increasingly harmless.

Calibrated Probing

A second and more concerning trajectory would involve activity that remains deliberately limited but displays patterns difficult to explain through spillover alone. The purpose of calibrated probing would not necessarily be to inflict physical damage. It could instead be to learn: how quickly NATO detects an intrusion, which aircraft are scrambled, where radar coverage is weaker, how national and NATO command structures coordinate, how frequently political leaders publicise attribution, and what level of repetition the Alliance tolerates before altering its response.

No single incursion would prove such a strategy. The evidentiary case would emerge through clustering.

Relevant indicators would include repeated appearances near the same military installations or surveillance gaps; routes inconsistent with nearby Ukrainian targets; incursions coinciding with major NATO exercises or deployments; recurring activity designed to elicit similar defensive reactions; and platforms carrying sensors or flight configurations more consistent with reconnaissance than accidental displacement. Patterns across several Allied states would be particularly important. What looks ambiguous in one national airspace may become more intelligible when compared with incidents elsewhere.

NATO’s own assessment increasingly emphasises that Russian airspace violations form part of a broader pattern of reckless and escalatory behaviour along the eastern flank, while Alliance officials have simultaneously identified sabotage attempts, cyber activity and other hostile actions as features of the changed European security environment.

The policy implication would differ substantially from managed spillover. If deliberate probing becomes the more convincing explanation, successful interception is no longer enough. NATO would need to deny the informational and political benefits of the activity and begin applying the graduated consequences outlined in the previous section. The objective would be to make repeated testing progressively less informative and more costly.

Infrastructure coercion

The third pathway would represent a shift from testing NATO’s reaction towards exploiting the vulnerability of the systems that sustain the eastern flank. The principal targets would not necessarily be military bases. They could be railways, border crossings, ports, energy infrastructure, telecommunications networks, undersea cables, airports and logistics corridors connecting NATO states with Ukraine and with one another.

There are already reasons to treat this pathway seriously. Poland has reinforced the Dorohusk, Medyka and Korczowa crossings following Russian strikes close to the Ukrainian side of the border. Recent attacks have struck a passenger train and other facilities close enough to Polish territory to interrupt crossings and require additional protective infrastructure. NATO and European governments have also intensified protection of critical undersea infrastructure. In September 2026, reporting based on Western security sources indicated that Allied forces had disrupted a suspected Russian operation involving sensitive undersea cables near Svalbard earlier in the year.

The defining characteristic of infrastructure coercion would be functional concentration. Events that appear heterogeneous at the platform level — a drone strike, cyber intrusion, GPS disruption or suspected sabotage — would begin producing effects against the same underlying function: transportation, energy supply, communications or military logistics. This is where cross-domain analysis becomes essential. A drone approaching a railway terminal may remain ambiguous. A cyberattack on the same logistics system may also be ambiguous. GPS interference in the surrounding region may be treated separately. If such incidents cluster temporally and functionally, however, the probability that they represent independent events declines.

The indicators to monitor are therefore not limited to the number of drones crossing borders. NATO should track target proximity, infrastructure function, temporal coordination and domain convergence. A simultaneous increase in physical intrusion, cyber disruption and sabotage attempts affecting the same logistics network would constitute a qualitatively different security problem from a series of unrelated airspace violations. This pathway would also have major implications for Article 4. Critical infrastructure may be pressured repeatedly without a single incident clearly reaching the armed-attack threshold. Yet the cumulative effect could impair NATO mobility, civilian safety or support to Ukraine. Infrastructure coercion therefore represents perhaps the clearest case for threshold management: consequences must accumulate as the campaign accumulates.

Threshold Crisis

The fourth pathway is the least desirable but strategically most consequential: an incident or accumulated campaign produces effects serious enough to force Allies to consider whether the security environment has crossed from sub-threshold coercion into armed attack.

Such a crisis need not begin with an unmistakable Russian decision to attack NATO. It could emerge through miscalculation. A drone or missile might strike populated territory and cause significant casualties; an attack on Ukrainian infrastructure could hit a major facility inside NATO territory; deliberate probing could encounter an unexpectedly forceful interception; or a campaign against critical infrastructure could generate physical destruction and loss of life that fundamentally alters its legal and political character.

The defining indicator would therefore be effects rather than platform type. A small and technologically unsophisticated system can generate strategic consequences if it kills civilians, disables critical infrastructure or creates sustained military disruption. Conversely, even a sophisticated aircraft briefly crossing a border may not constitute an armed attack if it produces no comparable effects and intent remains uncertain.

The most dangerous version of a threshold crisis would involve cross-domain clustering combined with casualties or major infrastructure damage. A physical attack accompanied by cyber disruption, electronic interference or sabotage would substantially strengthen the inference that NATO faced a coordinated campaign rather than an isolated accident. At that point, the distinction between Articles 4 and 5 would become a live political question rather than an abstract doctrinal debate.

The purpose of threshold management is precisely to reduce the probability of reaching this stage. A system that generates consequences before coercion becomes an armed attack gives both NATO and the adversary more opportunities to alter behaviour before escalation becomes difficult to reverse.

Reading The Pattern

These four pathways suggest that counting incidents is insufficient. NATO should evaluate the eastern flank through a broader set of indicators:

Indicators And Their Analytical Significance

• Frequency: Is activity becoming persistent rather than episodic?
• Geographical clustering: Are incidents concentrating around particular borders, bases or corridors?
• Flight behaviour: Are routes consistent with spillover or with reconnaissance and probing?
• Target proximity: Are military, transport, energy or communications assets repeatedly approached?
• Attribution confidence: How strongly can the platform, operator and command chain be identified?
• Temporal coordination: Do incidents coincide with exercises, political decisions or other hostile activity?
• Cross-domain convergence: Are air, cyber, electronic and sabotage activities affecting the same function?
• Effects: Are disruption, damage and casualties increasing even if individual actions remain limited?

No single indicator should determine NATO’s response. The important change occurs when several begin moving in the same direction.

The distinction between frequency and clustering is particularly important. Twenty unrelated incidents generated by a high-intensity war may be strategically less significant than five incidents repeatedly concentrated around the same railway corridor, airbase or undersea communications network. Similarly, attribution of one drone may matter less than evidence that different forms of hostile activity are converging against the same function.

Recent NATO adaptation suggests increasing awareness of precisely this informational challenge. In September 2026, the NATO Communications and Information Agency launched a new counter-unmanned-aircraft data initiative aimed at combining radar, radio-frequency, optical and acoustic sensor information through artificial intelligence to improve the detection, classification and identification of small drones. The significance of such efforts goes beyond technical interception. Better classification reduces uncertainty, and reducing uncertainty improves political decision-making.

Forecasting on the eastern flank should therefore avoid dramatic predictions about an inevitable NATO–Russia war. The more useful task is to determine whether the observable evidence continues to fit a model of managed spillover, begins to display the signatures of calibrated probing, shifts towards infrastructure coercion, or produces effects consistent with a threshold crisis. That distinction matters because each pathway requires a different response. Treating spillover as deliberate attack risks unnecessary escalation. Treating deliberate probing as accidental spillover creates opportunities for exploitation. Treating infrastructure coercion as a collection of unrelated technical incidents obscures the campaign-level effect. And waiting until a threshold crisis before imposing meaningful consequences would surrender precisely the strategic space that threshold management is designed to govern. The task for NATO is therefore not to predict the next incident. It is to recognise when the logic connecting incidents has changed.

Conclusion – NATO Does Not Need A Lower Article 5

NATO’s eastern flank is not simply experiencing more airspace violations. It is confronting a security environment in which individually limited incidents can accumulate into a persistent strategic burden. Drones crossing borders, strikes near logistical corridors, electronic interference, suspected sabotage and pressure on critical infrastructure do not all carry the same intent or legal significance. Yet they increasingly impose the same requirement on the Alliance: to detect, classify, interpret and respond under conditions of uncertainty.

The central challenge is therefore cumulative rather than episodic. What began as spillover from the war in Ukraine has developed into a broader problem of sub-threshold security management. Some incidents will remain accidental. Others will result from reckless military activity conducted too close to Allied territory. A smaller number may constitute deliberate probing or coercion. NATO cannot afford to treat these categories as identical. But neither can uncertainty over intent become a reason to ignore the strategic effect produced by repetition.

The Alliance has already adapted substantially. Eastern Sentry, stronger air policing, expanded surveillance, counter-drone investment and greater protection of critical infrastructure have reduced vulnerability and improved the speed with which incidents can be managed. These measures matter. They demonstrate that NATO is more capable of detecting and defeating low-end threats than it was when Russian incursions first became a sustained concern.

But operational competence is not the same as deterrence.

A system that successfully intercepts each drone while leaving the incentive to launch or tolerate the next one unchanged is managing risk rather than necessarily reducing it. The problem becomes more acute when inexpensive activity repeatedly forces expensive Allied responses. NATO must therefore address both sides of the exchange: lowering the cost of defence while increasing the expected cost of deliberate repetition.

This is why Article 5 should not be weakened, automatic or easier to trigger. Its value rests precisely on the seriousness and political weight of collective defence. Lowering the threshold would risk transforming ambiguity into escalation and could allow minor provocations to exert disproportionate influence over Allied decision-making. The stronger solution lies below that threshold.

The concept of threshold management developed in this article offers one way to think about that space. Its purpose is not to replace political judgement with automatic escalation. It is to ensure that recurrence changes consequences. Isolated and ambiguous incidents should primarily be denied and investigated. Repeated violations should generate collective attribution and a higher level of monitoring. Deliberate probing should lead to progressively stronger and preferably reversible countermeasures. Pressure on critical infrastructure should trigger coordinated military, political, economic and cyber responses. Only when effects reach the level of armed attack should Article 5 become the central question.

Such an approach would preserve ambiguity where NATO benefits from it while reducing ambiguity where an adversary may exploit it. Moscow should remain uncertain about the precise point at which an armed attack would generate collective defence. It should not, however, be confident that activity below that point will produce no cumulative consequence.

The distinction will become increasingly important if the war in Ukraine continues to shape the security environment beyond Ukraine itself. NATO’s task will not be to eliminate every incursion or prevent every accident. No air-defence architecture can achieve that. The more realistic objective is to prevent repeated ambiguity from becoming a strategic resource for an adversary.

NATO therefore does not need to make collective defence easier to trigger. It needs to make coercion below collective defence harder to exploit. The credibility of deterrence on the eastern flank will increasingly depend not only on what NATO promises to do once Article 5 is crossed, but on whether it can impose order, proportionality and accumulating consequences in the difficult space before that threshold is ever reached.