Something is spooking Europe.

This week, Belarus, Russia’s closest military ally, conducted large exercises near the Suwałki Gap, the narrow corridor connecting the Baltic states to the rest of NATO. The geography that has long made it one of NATO’s most vulnerable points.

French President Emmanuel Macron summoned France’s political leaders for a confidential intelligence briefing, then announced new measures to protect critical infrastructure from intensifying Russian hybrid attacks.

Polish Prime Minister Donald Tusk went further, warning that Russia may launch drone or missile attacks against NATO countries and disguise them as accidents.

The UK has told citizens to start preparing for the possibility of war, and to stockpile canned foods and drinking water. Its chief of defense said Britain faced the “most dangerous” period in decades.

Germany is preparing its hospitals for a possible Russian attack. Its Chancellor, Friredrich Merz, warned of “decisive moments” in “defending the freedom and peace of our continent.”

Even the famously neutral Switzerland adopted a new national security strategy calling for greater resilience and an ability to defend itself against an armed attack.

President Donald Trump, meanwhile, suddenly announced progress toward establishing a permanent US Army base in Poland. Two weeks ago, he dispatched the CIA Director on a secret mission to warn Moscow against attacking NATO territory.

Countries on both sides of the Atlantic seem alarmed that Russian President Vladimir Putin might soon widen his war beyond Ukraine.

The United States got a glimpse of the threat this week.

Federal prosecutors charged five people allegedly working with Russian intelligence in a network plotting attacks and murders around the world, including inside the US. Prosecutors also allege the network commissioned attacks against civilian and military infrastructure in European countries.

The charges fit a broader pattern Europeans have confronted for years: cyberattacks, sabotage, arson, drones and attacks against critical infrastructure. Last month, Germany blamed Russia for a drone attack at a civilian airport.

The strategic challenge for NATO is that Russia can conduct such operations without clearly crossing the threshold into conventional war.

Fly a drone into NATO airspace, disrupt an airport, sabotage infrastructure, conduct murders through criminal networks — all deniable “gray zone” operations.

Putin uses ambiguity and denial as a weapon, testing NATO resolve, while recognizing that NATO is divided on how best to respond, or to respond at all.

But what if Putin decides to go further?

The Suwałki Gap offers one answer.

It is a roughly 65-mile border between Poland and Lithuania and the only land connection between NATO’s Baltic states—Lithuania, Latvia and Estonia—and the rest of NATO.

Imagine a crisis in which Russia, perhaps operating with Belarus, moves forces into a portion of the corridor. It’s not an explicit invasion of a NATO state, such as tanks rolling into Poland. Putin would claim a limited incursion if he said anything at all, and that Russia did not seek escalation.

Russia would also claim through non-official channels that a NATO response risked nuclear war.

The purpose is less about the Gap and severing NATO’s Baltic flank, than undermining NATO and its mutual defense commitments altogether.

Would 32 NATO countries agree to fight Russia to recover a small strip of territory? Would an American president risk escalation with a nuclear power over Lithuania? Would Germany and France?

Nobody can say for certain, and that plays into Putin’s hands.

NATO’s Secretary General Mark Rutte was asked this year about a war game involving precisely such a Russian move against the Suwałki corridor. His response was that NATO regularly exercises against such scenarios and that an attack would produce a devastating response.

Does Putin believe him? With Washington at loggerheads with its closest allies, including Canada — perhaps not.

Ukraine has provided Europeans with a brutal tutorial in what modern war requires.

Countries need more than military materiel. They need electrical grids that can survive attacks, hospitals capable of handling mass casualties, redundant communications, functioning transportation networks and governments able to operate under sustained pressure.

Most importantly, they need societies capable of absorbing costs without collapsing or polarizing.

This week, NATO publicly released updated requirements for national resilience, covering everything from continuity of government and energy supplies to food, water, health care and communications during “crisis and conflict.”

NATO officials are studying how Ukrainians have kept essential services functioning under sustained Russian attack.

It’s an extraordinary turn of events. Ukraine is no longer simply receiving lessons from NATO — NATO is also receiving lessons from Ukraine.

In doing so, the alliance aims to deter not just by promising a devastating response to aggression — but by demonstrating to Putin that his aggression won’t work.

Preparedness itself becomes part of deterrence. That is one reason for the slew of headlines this week.

There is also an American factor behind Europe’s urgency.

The US is simultaneously deployed in Europe, fighting Iran in the Middle East and deterring China in the Indo-Pacific. Air defenses, naval forces, intelligence platforms, and depleting munitions cannot be everywhere at once.

Trump has done much to strengthen NATO, but his commitment to its mutual defense clause is uncertain.

Moscow knows this.

Russia remains bogged down in Ukraine. But Putin does not need another invasion on the scale of Ukraine to challenge NATO. He can move up an escalation ladder: sabotage, cyberattacks, assassination plots, drone or missile attacks — and then a limited military operation designed to test whether NATO still means what it says.

Ask yourself whether Trump would go to war with Russia over a patch of land called the Suwalki Gap? Would Americans be ready for such a thing? The question answers itself.

And that alone undermines deterrence.

For three decades after the Cold War, Europe allowed national preparedness to atrophy. Risks of war seemed remote. Spending on militaries and recruitment plummeted. Civil defense was a relic of the past. And the US was presumed to be there in a pinch.

That era is over.

Europe is preparing for war because its leaders increasingly understand an uncomfortable truth: deterrence depends on what Putin believes, and America’s response to a widening war would be uncertain.

The Belarusian exercises near the Suwalki gap are raising alarms because, despite Russia’s struggles in Ukraine, the deterrence equation has atrophied.

The best way to prevent a widening war is to make certain Putin believes it will fail.

And judging by the urgent moves now underway across European capitals, there is growing concern that he may believe the opposite.