A Polish soldier has died defending the country’s border with Belarus. But the cause of his death is not a traditional military assault: the man, Mariusz Sitek of the 1^st Armored Brigade, died after being stabbed by a migrant who had arrived at the border with the help of Belarusian authorities.

The killer was one of the 17,000 whom the Belarusian government has transported to the Polish border this year alone as part of its continuing campaign to destabilize the European border with non-military means.

This is so-called grayzone aggression and it’s getting much worse as Russia and its allies step up campaigns across the European continent. It is causing serious alarm among governments and security officials, and as yet, there are no agreed methods to counteract it.

The audacity of the attacks is growing. The Polish incident claimed a human life, while a recent cyberattack on several London hospitals may cause the loss of further lives. The Czech premier said on June 10 that an arson attack in Prague was likely the work of Russia, while Polish premier Donald Tusk said nine men linked to Russian intelligence were arrested for planning sabotage campaigns. European governments, and NATO, now face the acute dilemma of how to respond.

NATO representatives are fond of saying that the alliance exists to defend “every square inch” of alliance territory. But last month, a migrant attached a knife to a long pole and thrust it through the border fence separating Poland and Belarus. It crossed the threshold of NATO territory and pierced the lung of the 21-year-old Mariusz Sitek. The badly wounded soldier was taken to a hospital, and on June 6 he succumbed to his injuries.

The fence separating Poland and Belarus consists of steel tubes topped by barbed wire. It’s a recent installation; until 2021, the two neighbors maintained mostly collaborative relations at the border. But that year, Aleksandr Lukashenka’s dictatorship decided to destabilize the EU and NATO, and it did so with a novel form of grayzone aggression.

All of a sudden, migrants from the Middle East, Afghanistan, and elsewhere began appearing at Belarus’s borders with Latvia, Lithuania, and Poland, delivered there with the help of Belarusian authorities, which had also conveniently begun issuing large numbers of tourist visas in traditional migrant countries.

Lukashenka’s obvious plan was to create an artificial migration crisis that would cause mayhem in the three countries and pit them against Brussels and western Europeans, who have traditionally advocated for a lenient approach to asylum seekers and other migrants. Europe getting bogged down by a divisive refugee crisis, and three countries struggling to keep their borders intact: it was a brilliant plan, especially since it entailed virtually no risk for Belarus and its Russian ally.

It didn’t work. Latvia, Lithuania, and Poland took a tough approach to the issue, which included erecting border fences, and while some in the EU initially criticized the heavy-handedness, the complaints quickly subsided as the brazen nature of the Belarusian regime’s behavior became apparent.

But the border fences have clearly not been enough to protect NATO territory, as Sitek’s death illustrates. While a NATO soldier has been killed as the result of a calculated operation by a hostile neighbor, any response to the attack is much more fraught than it would be if it were a traditional military assault. Poland’s Foreign Minister Radek Sikorski has demanded the killer’s identification by Belarusian authorities and his return for trial, so far to no avail.

This is precisely what grayzone aggression is designed to do — hurt your enemy and leave them unable to respond. There’s simply no rulebook on what to do. Responding with military means would be needlessly escalatory, while not responding at all would allow the aggression to continue. How about responding with an eye for an eye? In most cases, that’s not possible, because liberal democracies try to uphold certain ethical standards. No one seriously imagines a Western democracy would recruit a migrant to stab a Belarusian soldier to death in retaliation.

Or consider the cyberattack that hit several London hospitals on June 4, rendering them unable to conduct operations, blood transfusions, and much else.

Ciaran Martin, former CEO of the UK National Cyber Security Centre, attributed the attack to Russian cyber gang called Qilin, which operates “freely from within Russia.” So far, no one has died as a result of the attack, but such is the chaos that resulted that the UK National Health Service (NHS) is now appealing to people with the O blood type (which is safe for all patients) to donate as a matter of urgency.

The NHS is the Western world’s largest national health service and is now experiencing a blood shortage as a result of a cyberattack supported, or at the very least condoned, by a hostile state. Another Western country loses a soldier as a result of machinations by a hostile neighbor. Poland, Lithuania, the Czech Republic, and the UK have all suffered suspected Russian-linked arson attacks.

That’s the harm grayzone aggression can cause. Yet it remains devilishly difficult to find a suitable response to it. That’s why my book about grayzone aggression is called The Defender’s Dilemma.

But the fact that grayzone aggression is extremely hard to punish doesn’t mean it should be tolerated or ignored. Especially now that the grayzone has cost the first NATO soldier his life.

The West needs answers, and quickly.

Elisabeth Braw is a Senior Fellow at the Atlantic Council.

Europe’s Edge is CEPA’s online journal covering critical topics on the foreign policy docket across Europe and North America. All opinions are those of the author and do not necessarily represent the position or views of the institutions they represent or the Center for European Policy Analysis.

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