Russian drones that flew into Poland last week were almost certainly not there by accident. This was a probe of NATO’s defences, another example of how coercive revisionist powers escalate within the grey-zone spectrum to test democracies’ resolve.

In sending the uncrewed aircraft over Poland, Russia was pushing against the thickest part of NATO’s shield, a country that is deep into sweeping military rearmament and modernisation—and one that stands strongly against the Kremlin in support of Ukraine. The question is whether NATO can deter such challenges. How it handles this episode will resonate far beyond NATO’s Eastern Flank.

Around 19 Russian drones entered Polish airspace overnight on 9 and 10 September, some coming through Belarus. NATO fighters on quick-reaction alert shot down at least three. This was the first engagement between NATO aircraft and Russian drones over allied territory. Polish Prime Minister Donald Tusk warned that ‘this situation brings us the closest we have been to open conflict since World War II.’

Before last week’s intrusion, violations of Polish airspace had looked unintentional, as single drones crashed just inside the border, often discovered by civilians. Belarusian helicopters briefly entered Polish airspace during exercises in 2023. A Ukrainian surface-to-air missile struck the Polish border village Przewodow in 2022.

By contrast, the latest incident was a significant intrusion by a small swarm, with one drone reaching as far as Mniszkow, a village more than 250 km inside Poland. The incident disrupted civil aviation and triggered consultations between NATO members under article 4 of the treaty.

Polish officials say the incursion was intentional, rejecting Belarus’s claim that the aircraft had lost their way due to jamming of satellite navigation signals. Moscow said it had planned to attack no targets in Poland, and it pointed to the drones’ maximum range of 700 km. But Poland is less than 700 km from Russia, and some reports indicate several of the uncrewed aircraft were specially configured with extra fuel tanks that extended their range. These alleged modifications, combined with the scale of the incursion, point to intent.

Poland fields the largest standing army in Central and Eastern Europe, and its defence spending as a fraction of GDP is the highest in NATO, around 4.7 percent. Its ongoing modernisation program includes advanced new air-defence systems and fighters and expanded armoured units, with plans to increase troop numbers

The country was likely targeted because of its proximity to Belarus and Ukraine, with Russia being able to plausibly claim this was just another instance of unintentional spillover from the war. Relations between Warsaw and Moscow are worse than at any time since the end of the Cold War, and Russia is seeking to punish Poland for its strong support of Ukraine. Poland serves as the main logistics hub for external support of Ukraine’s war effort. Its southeastern city Rzeszow is a major transit point for Western military aid.

This incident can be seen as part of a broader Russian hybrid warfare campaign that has included cyberattacks, disinformation and suspected sabotage. Poland has been a prominent target for this since the war in Ukraine began.

The drone incursion also shows that the airspace of even an unusually steadfast NATO member can be penetrated cheaply and easily. If Poland’s defences can be tested in this way, then confidence in NATO’s general ability to deter and respond is weakened.

In response to the incursion, NATO has launched Operation Eastern Sentry, deploying more air-defence equipment, including fighter jets and a frigate, to its eastern flank. Moscow benefits from imposing costs on NATO members as they expensively intercept cheap drones.

How Europe further responds to this incursion, and what consequences are imposed on Moscow, will set a precedent. If Russia can get away with such violations, it pushes out the threshold of the grey-zone spectrum of provocations less than war.

For NATO’s Indo-Pacific partners, particularly Australia, this offers a valuable lesson in how cheap drones can probe even robust air defences and erode deterrence.