Newt Gingrich called Donald Trump’s servile performance alongside Vladimir Putin in Helsinki this week the “most serious mistake” of his presidency. Gingrich only had to wait a few hours for the president to top himself.

After he spent an hour emboldening Russia’s autocratic president by insisting that “both sides” are to blame for Moscow’s attack on private American interests in 2016, Trump sat down with Fox News Channel’s Tucker Carlson for an interview in which he questioned the value of America’s mutual defense commitments abroad. “Why should my son go to Montenegro to defend it from attack?” Carlson asked, citing specifically the Balkan nation that ascended to NATO membership last year. Rather than offer a coherent defense of the post-World War II consensus, articulate how the Atlantic alliance advances U.S. interests, or expand on the threat Russia poses to its member states on the periphery, Trump seemed to accept Carlson’s premise.

“I’ve asked the same question,” the president replied. He called Montenegro a “tiny country” populated by “aggressive people” who, like America apparently, might just deserve a punch in the nose from Moscow. “They may get aggressive, and congratulations you’re in World War III,” Trump said. This is a remarkably dangerous sentiment for any American official to express, much less the president of the United States.

Putin has spent the last decade communicating his willingness to use military force to change borders and to halt NATO’s eastward expansion. His invasions of Georgia and Ukraine, followed by the functional or formal annexation of territory in those countries, represent the greatest threat to the American-led postwar order in Europe in the last half-century. Putin regularly tests the Western alliance’s willingness to defend itself. He has executed crippling cyber attacks on American allies. He has engineered border incidents and kidnapped soldiers to use as propaganda tools. Even “aggressive” Montenegro has been the target of Putin’s belligerence. To prevent Podgorica’s ascension into NATO, Russian military officers executed a failed operation in 2016 designed to topple Montenegro’s democratically elected government, assassinate its prime minister, and install an anti-Western regime.

So why would Putin go to all that trouble over a “tiny country?” Not only is Montenegro of strategic value insofar as it had the last non-NATO deepwater port on the Western Mediterranean, but NATO membership takes a country off the board for Russia. Both Ukraine and Georgia were on track to ascend to NATO membership, but weak-kneed Western European leaders blocked those efforts in an attempt to curry favor with Russia. The stall was just long enough for Moscow to interfere in their politics and, eventually, destabilize them militarily. Neither nation will ascend to NATO now, lest the alliance take ownership of Russia’s aggression toward them. Moscow has effectively blocked those sovereign nation’s aspirations to orient themselves westward. It tried to do as much to Montenegro, but it failed.

Moscow is a declining power, and that is precisely what makes it so dangerous. Russia is aware that time is not on its side. It cannot afford conventional conflict with the West, but it has proven willing to test its boundaries. Donald Trump’s willingness to send ambiguous signals about America’s firm defense commitments risks encouraging Moscow to take even more provocative actions. And that could bring about a terrible crisis.

Putin’s objective is to tear the alliance apart, but he cannot do so through direct confrontation; that is a conflict he would lose. The only way he can achieve his goal is to render the alliance moot, and the quickest way to do that would be to engineer a crisis on NATO’s periphery that forces one of its smaller members to call for help. If Estonia asked the West to go to war with Russia in its defense, would we? That’s an open question. If Tallinn invoked NATO’s mutual defense provisions as America did in 2001 and no one came to its aid, the Atlantic alliance would essentially cease to exist, and the postwar order in Europe would go with it.

Communication is the essence of deterrence. An adversary who doesn’t know what you are prepared to defend and what you’re willing to sacrifice will test you. If those boundaries are unclear, that adversary runs the risk of crossing a line that provokes a disproportionate response. Once that cycle of cascading and reciprocal reprisals begins, it becomes difficult to stop.

That is the essential value of NATO. It is a vehicle for deterrence. Its borders are clear, its interests are well defined, and the consequences for violating either are dire. Not only has the Atlantic alliance helped keep the peace among its formerly conflict-prone members, it has raised the cost of overt Russian interference in Western affairs to a prohibitive point. NATO remains a bulwark against Russian antagonism, and its mutual defense guarantees ensure that Americans who volunteer to defend U.S. interests at home and abroad will never be asked to sacrifice their lives in Europe again. An American president should be expected to know that.

In May of 1939, the French socialist newspaper L’Œuvre articulated the essential logic of appeasement when it asserted that the Nazi’s territorial demands on the Polish Republic’s port on the Baltic Sea were not worth contesting. Its arguments were cowardice cloaked in false bravado and cheap nationalism, but it nevertheless put those who were inclined to confront Hitler on the defensive. A slight variation on the title of that article soon became the anti-war slogan adopted by pacifist and isolationist movements on both sides of the Atlantic: “Why die for Danzig?” The Second World War began four months later all the same. Tens of millions would die because the spine required to confront and contain the Nazi menace was in short supply.

So why die for Montenegro? The goal of American strategy in Europe is that, through proactivity and deterrence, no one will. When we fail to communicate to aggressors that their actions will result in consequences that they cannot endure, we run the risk of inviting another terrible conflict. It is one of history’s great ironies that those who claim to be war’s greatest opponents have such a lamentably consistent track record of bringing it about.