Testing the Red Line: Russian Drones and NATO's Article 5

The Unanswered Question Over European Skies

The skies above NATO member states no longer appear entirely peaceful. After Russia repeatedly violated the airspace of the Baltic states, Poland, Romania, Denmark and Germany in September, the Alliance began to seriously consider the exact limits of its ultimate security guarantee, Article 5. 

The events of 9-10 September caused a stir and led to a series of Russian manoeuvres. Nineteen drones participating in a large-scale air raid on western Ukraine deeply invaded Polish airspace. Polish F-16s and Dutch F-35s took off to intercept them and shot down four drones. Although no casualties were reported, debris from the downed drones rained down on eastern Poland, causing material damage and bringing the physical reality of the war in Ukraine to NATO territory. 

Polish officials called the incident a deliberate ‘test’ of NATO's resolve. Warsaw invoked Article 4 of the North Atlantic Treaty, which obliges allies to hold emergency consultations. Poland then publicly proposed the creation of a NATO no-fly zone over western Ukraine to prevent future incursions. Given the West's continued restraint in commenting on previous Russian missile incursions into NATO territory, this is a significant and encouraging step, at least on the part of the government of the country that was attacked. Later, Polish Prime Minister Donald Tusk issued a statement saying that Poland would ‘shoot down flying objects that violate our territory.’ 

However, this was not an isolated challenge. The provocations continued on 19 September, when three Russian MiG-31 fighter jets violated Estonian airspace over the Gulf of Finland, prompting another scramble of NATO fighter jets and swift diplomatic condemnation, which Russia, as usual, denied. A day later, another massive attack by Russian drones and missiles near the Polish border again forced Polish and allied fighter jets to scramble as a defensive measure.

The campaign then spread north. There was a serious escalation on the night of 22 September when several large drones forced Copenhagen Airport in Denmark to close for four hours. Danish Prime Minister Mette Frederiksen called the intrusion ‘the most serious attack on Danish infrastructure to date,’ noting that Russia's involvement could not be ruled out. Similar drone sightings also disrupted operations at Oslo Airport in Norway, indicating systematic attempts to probe and disrupt critical infrastructure across Scandinavia.

Is this series of events really a prelude to a major war? Not necessarily. Ukrainian President Volodymyr Zelensky has also suggested that such incidents may be part of Moscow's strategy to reduce European military aid to Ukraine. In these critical times, we need to consider various scenarios. 

The “Musketeer Clause”: Deconstructing Article 5

Article 5 of the NATO Treaty is somewhat reminiscent of the Musketeers' oath: ‘One for all, all for one.’ It enshrines the principle of collective defence: an armed attack against one member is considered an attack against all. However, Article 5 does not apply as soon as hostile drones appear on the Alliance's border. Determining whether an action constitutes an ‘armed attack’ is a matter of political assessment rather than a specific legal definition. The treaty deliberately leaves the procedure for action vague. According to it, each member will take ‘such measures as it deems necessary,’ which can range from further sanctions and diplomatic pressure to targeted military strikes or a full-scale collective military response. This ambiguity gives the alliance flexibility in its response. But is this really in the interests of collective security? On the one hand, the theory of deterrence teaches us not to respond to aggression with aggression, because an immediate declaration of war, such as the one currently against Russia, can only lead to escalation and entail avoidable losses. However, the security dilemma lies in whether it is necessary to wait until NATO citizens suffer from Russian attacks. 

Article 5 has been invoked only once in NATO's history — by the United States after the 9/11 terrorist attacks. The threshold for invoking the article is high and applies only in the case of ‘the most serious attacks.’ But the weakness lies in who determines what constitutes a ‘most serious’ attack. 

Four Scenarios That Could Trigger Article 5

Scenario 1: An invasion causing mass casualties or critical damage

If a Russian missile or armed drone evades air defences and strikes a populated area, such as a school or hospital, or critical infrastructure, such as a power plant, resulting in significant casualties, it would be almost impossible to dismiss this as an accident. The scale of the loss of life and damage would likely be considered an ‘armed attack,’ forcing the Alliance to invoke Article 5.

Scenario 2: Downing of a NATO manned aircraft

The shooting down of a Polish or other allied aircraft by a Russian fighter jet during an interception in Polish airspace would be a direct act of military aggression against a NATO target. The death of allied military personnel in such an incident would be an unambiguous act of war, making the invocation of Article 5 highly likely.

Scenario 3: A Deliberate, Sustained Attack on Polish Territory

If Russia launches a wave of drones or missiles whose flight path clearly indicates that their targets are in Polish territory and are not the result of attacks on Ukraine, the apparent intention to strike NATO territory will remove any ambiguity. This would be an evident reason to invoke Article 5. Isn't this precisely what we saw on 9-10 September? As we can see, both Poland and other members of the Alliance were very cautious and afraid that their rhetoric would further ‘scare’ Russia, so that it would not attack the West. However, given that the West has been supplying Ukraine with weapons on a massive scale for more than three years, it remains unclear what has caused such timidity towards a terrorist state like Russia. 

Scenario 4: A Hybrid Attack with Attributed Casualties

NATO has recognised that severe cyber attacks could trigger the application of Article 5. Suppose a major cyberattack, likely linked to Russia, disables a critical part of infrastructure, such as the power grid or air traffic control system, directly leading to fatalities. In that case, the link between the attack and the loss of life will be a key factor in deciding whether to invoke the collective defence clause. 

We have already seen how the power grids of France, Spain and Portugal were affected a few months ago. To the best of our knowledge, France was the first and only country to publicly announce Russian involvement in the cyberattack.

Walking a Tightrope Between Deterrence and De-escalation

Of the four scenarios we might imagine, two have already come to pass, mercifully without loss of life. This poses an agonising question: must NATO wait for a tragedy on its own soil before it truly confronts the threat? It is, of course, entirely understandable that any direct military response risks a broader war and even greater bloodshed. This is the very puzzle our leaders must now solve: how to act decisively to prevent future losses, both economic and human, without triggering a catastrophic escalation. In truth, the war is already here. To stop it from spilling beyond its current borders, NATO's primary goal must be a consolidated, constructive effort inside Ukraine—with the maximum possible investment in weapons, training, and support to bring this war to a definitive end, in Ukraine.

These drone incursions have ignited a rather fraught debate within the Alliance over what deterrence truly means in this new era. At its heart, the discussion is whether to give the order to shoot down Russian assets that violate allied airspace. On one hand, the frontline states, such as Poland and the Baltic nations, argue that the current policy of restraint is simply failing. Their perspective is grounded in classic deterrence theory, which tells us that a threat is only credible if there is an apparent willingness to act upon it. By repeatedly allowing drones and jets to cross the border without a firm, physical response, NATO is signalling to Moscow that its 'red line' is, in truth, relatively porous. They contend that only by downing these objects can the Alliance re-establish a credible deterrent and prevent Russia from testing the limits still further. 

On the other hand, some allies remain acutely wary of the classic 'security dilemma'. The architects of this theory, writing in the shadow of two world wars and the dawn of the nuclear age, would remind us of a paradox at the heart of international relations. In a world of uncertainty, the steps one state takes to bolster its own defence are often seen by its rivals as inherently threatening. This can trigger a tragic spiral of hostility, where purely defensive intentions are misread as aggressive ones, leading both sides to escalate their postures until conflict becomes unavoidable. 

Applied to today's crisis, the act of shooting down a Russian drone, while perfectly justifiable as a defence of sovereign airspace, would be entering uncharted territory. In fact, this is precisely what Russia already uses in their propaganda. Is it a simple act of policing a border, or is it the first shot in a direct conflict between nuclear powers? The ambiguity is precisely the point. The Kremlin would undoubtedly portray it as an act of overt aggression, regardless of NATO’s intent. The fathers of the security dilemma would stress that with the nuclear dimension, the stakes of such a miscalculation are almost unimaginably high. Indeed, what we are witnessing is Russia deliberately exploiting this very dilemma. By using low-cost, often unarmed drones to conduct these probes, Moscow forces NATO into an impossible choice: either do nothing and appear weak, or react forcefully and risk triggering the very escalatory spiral that these foundational thinkers warned us about.

Each time a drone crosses the line without consequence, a dangerous precedent is set, and the Kremlin's interpretation of NATO's passivity invites more brazen provocations. The current strategy is failing to deter. To prevent a catastrophic spillover, the West must adopt a different approach. This requires crafting a courageous and proactive deterrence posture that is unified, unambiguous, and understood in Moscow. It is not about seeking conflict, but about re-establishing the sanctity of the red line to prevent a much larger war.