Donald Trump every so often suggests he’d like to pull the United States out of the the North Atlantic Treaty Organization, knowing full well that would be the end of NATO as we’ve known it for almost 70 years. And such hints, the most recent of them last week at a summit in Brussels, must make Vladimir Putin’s crocodile smile spread from ear to ear.

Indeed, going into the Helsinki summit with Trump on Monday, Putin is surely aware that it was not necessary for Trump to pull the U.S. out of NATO in a formal sense for him to harm the alliance — and benefit Russia. Official statements notwithstanding, merely threatening to walk away from the Atlantic Alliance has been deeply destabilizing, and no amount of frantic damage control by Trump underlings is likely to fix that.

“In casting doubt on America's commitment to defend NATO countries against attack, Trump is forcing these countries to begin preparing to go it alone or realign themselves under a new European-based alliance to provide collective defense without U.S. support,” Bruce Blai r, a Princeton University nuclear expert, tells The Daily Beast.

If Trump actually did withdraw, Europe would become right away much more vulnerable to Russian nuclear attack or, more likely, intimidation. But immediate nuclear fire isn't the only danger.

More realistically, the Americans leaving NATO would force European countries that currently lack nuclear arms to toss aside the 1968 Non-Proliferation Treaty and rush to acquire them, all in order to deter the Russians without the Americans' help.

The treaty's disintegration could then lead to countries all over the world pursuing their own nukes. "Trump is increasing the chances of the bomb spreading and the key treaty keeping the lid on such proliferation collapsing," says Blai r.

Unconstrained nuclearization is one nightmare scenario that is becoming increasingly plausible as Trump escalates his criticism of the 69-year-old North Atlantic alliance. For three quarters of a century, American nukes have made it unnecessary for many European countries to possess nukes of their own.

Because of that, these countries could safely sign on to the Non-Proliferation Treaty, bolstering international efforts to limit nuclearization all over the world. "Among the benefits of NATO, a key one is that it has helped to prevent the spread of nuclear weapons," Kingston Reif , an analyst with the Arms Control Association, told The Daily Beast.

But that was long before Trump’s rise as a political force. In 2017, the former reality-T.V. star declared NATO "obsolete." In Brussels on July 11, Trump again questioned the organization's usefulness. "What good is NATO?" he asked. The next day at a meeting of NATO leaders, Trump threatened that he might "do his own thing" if alliance members didn't immediately increase their military spending.

Trump's words sent a chill through European capitals. The United States is by far the biggest military spender in NATO and, according to Mark Simakovsky, a fellow with the Atlantic Council's Eurasia Center, the "glue" that holds the alliance together. "Don't forget, there are huge divisions in Europe," Simakovsky said.

NATO's 29 member states range from illiberal Turkey, Hungary and Poland on the alliance's eastern flank to stalwarts France and Germany at the heart of the continent and the restive United Kingdom in the west.

At present just two non-U.S. NATO states – the U.K. and France – possess nuclear weapons. France fields around 300 nukes. The U.K., around 215. By contrast, the United States maintains an arsenal of no fewer than 3,800 atomic warheads, only slightly fewer than Russia possesses. The U.S. military keeps 180 warheads in Europe for use by its own forces and the forces of certain NATO members, most notably Germany.

Practically speaking, America is Europe's nuclear shield.

Under Article V of the NATO charter, an attack on any NATO state represents an attack on every other state – and the alliance is obligated to respond. That applies to a nuclear strike as well as conventional attack. If Russia nuked, say, Lithuania or Poland, the United States would be obligated to nuke Russia right back.

That mutual nuclear threat has helped to keep the peace in Europe since the Soviet Union exploded its first atomic bomb in a test in 1949, the same year as NATO's founding.

But with France and the U.K. possessing so few atomic warheads compared to Russia, deterrence in Europe could begin to collapse without American nukes. And that risk could drive European countries to create their own, more powerful deterrents – either collectively or individually.

"The loss of U.S. reliability to deter aggression against NATO Europe would prompt France and the U.K. to expand their nuclear capabilities and Germany and other non-nuclear countries to consider building their own nuclear arsenals despite strong public opposition," Blai r said.

Some European officials are already thinking in those terms. In 2017, Jaroslaw Kaczynski, chairman of Poland’s ruling Law and Justice Party, called for Europe to build up a combined nuclear arsenal as powerful as Russia's own arsenal. Conservative German parliamentarian Roderich Kiesewetter endorsed the idea.

If the United States were to leave NATO, Europe could build its own deterrent under the umbrella of a diminished NATO structure, or opt for a new structure based on the European Union. In the last decade or so, the E.U. has begun to establish a rudimentary military organization, but has deployed troops only rarely – and then mostly in Africa on peacekeeping duties.

The realignment could get complicated. Albania, Canada, Iceland, Norway and Turkey are in NATO, but aren't in the E.U. Austria, Finland, Ireland, Malta and Sweden are in the E.U., but aren't in NATO. Ireland, for one, is strictly opposed to nuclear weapons. "There are European Union members with nuclear capabilities, but how those capabilities would be employed outside of a NATO context – it's never been fleshed out," Simakovsky said.

For Trump to even threaten to pull back America's atomic umbrella is dangerous, Simakovsky said. "What it encourages is instability."

And that instability – and the resulting mistrust between former allies – plays into the hands of Russian president Vladimir Putin. It could even, in the most extreme scenario, tempt Putin to launch his own limited nuclear strike in the context of a wider war in Europe.

In the last decade Russia has invaded two of its European neighbors – Ukraine in 2014 and the Republic of Georgia in 2008. Neither Georgia nor Ukraine is a full member of NATO, although both countries have signalled their desire to join the alliance.

The Eastern European states of Lithuania, Latvia and Estonia – all former Soviet republics – and Poland, formerly a Soviet satellite, are NATO members and view themselves as the main targets of Russia's aggression. This year, Russia deployed nuclear weapons to Kaliningrad, a Russian enclave on the Baltic Sea between Poland and Lithuania.

The Trump administration criticized the Russian deployment as "destabilizing." But the greater threat of destabilization comes from the administration itself as it continues to dismantle rhetorically a security structure that has preserved the peace – and deterred nuclear war – in Europe since 1949.

"If Putin somehow decides to cross the nuclear threshold, it won’t be because he thinks we don’t have enough nuclear weapons," Reif said. "It will be based on a political calculation that he has a greater stake in the conflict and we and our allies won’t be willing to run the risk of escalation."

The alternative is only less awful. That, in the absence of America's nuclear guarantee as part of a transatlantic alliance, Europe might build up a large nuclear arsenal of its own and supercharge global atomic proliferation. "Nothing would do more to cause nuclear anarchy than wrecking NATO," Blai r said.