In mid-July, Donald Trump startled American allies and adversaries with a single startling statement: if elected president, he wouldn’t necessarily defend America’s allies in NATO if they were attacked by a foreign power. This extended, Trump said, to the Baltic countries right on Russia’s border — the very countries Russia might conceivably invade.

Now that he’s going to be the president, we’re about to see if Trump is willing to play with literal fire in Eastern Europe.

The NATO alliance was founded on a promise that an attack on one NATO country is an attack on all. Trump is directly undermining this promise.

The consequences are hard to overstate. He is trashing one of the foundations of the postwar European order, which has helped guaranteed peace on the continent for 70 years. And by equivocating on whether he would defend the Baltics, he creates a dangerous amount of uncertainty among Russians as to how seriously the US takes its NATO treaty commitments — the kind of uncertainty that could conceivably spark an actual conflict between the US and Russia.

Trump has made headlines throughout the campaign by publicly praising Putin and expressing policy positions — like hinting that he’d prefer if Syrian President Bashar al-Assad, a Russian ally, retain power — that closely align with the Kremlin’s preferences. But as president, he will be under tremendous pressure from our allies adhere to our traditional commitments to NATO. This creates a situation where Russia might not believe Trump would defend a NATO ally, but he actually would in a crisis. Those are the conditions under which a catastrophe becomes most thinkable.

To be clear: This nightmare scenario is still unlikely. That doesn’t make it any less scary. And that doesn’t make it something we should ignore with Trump just weeks away from moving into the White House.

What Trump said

In the July interview, the New York Times’s David Sanger asked Trump if he would defend our allies in NATO and East Asia. Trump said he wasn’t sure, that he would only be certain to defend countries that he thought had paid the United States enough money.

“If we are not going to be reasonably reimbursed for the tremendous cost of protecting these massive nations with tremendous wealth … then yes, I would be absolutely prepared to tell those countries, ‘Congratulations, you will be defending yourself,’” Trump told Sanger.

This is classic Trumpism. Throughout the campaign, he repeatedly insisted that American alliances don’t help the United States that much, that America is owed much more from its allies than it receives. As a result, he says, the US needs to back away from its alliance commitments. This is a consequence of his basically transactional worldview, which sees all foreign policy issues through the lens of what America gets out of it in cash terms.

The problem, however, is that the US is treaty-bound to defend its NATO allies. When NATO was created in 1949, it was built around a promise that an attack on one country would be considered an attack on all countries. You invade Poland, you start a war with the United States.

Now, NATO doesn’t have the power to force the United States or any other power to defend anyone else. Article 5, the provision in the NATO treaty that provides for collective self-defense, isn’t binding on America in the way the US Constitution is.

Instead, Article 5 works by credible commitment: If the United States signals that it is fundamentally committed to the NATO treaty, then it sends a signal to Russia and other hostile powers that the US will abide by the term of its agreements. This deters them from launching wars or any other kind of military adventurism in an American-aligned state.

This is most relevant in the Baltic NATO states: Estonia, Latvia, and Lithuania. These countries were former Soviet republics, and Putin seemingly believes they still ought to be Russian possessions. He has routinely screwed with them: kidnapping an Estonian security officer in 2015, sending Russian warships into Latvian waters 40 times in 2014, and repeatedly buzzing their airspace with Russian jets.

These countries’ best hope is their NATO membership: the idea that Putin would never do in these countries what he’s doing to Ukraine, because that would mean war with the United States. But when Sanger asked Trump specifically about his feelings on Baltic allies, he said openly that he wouldn’t defend them.

Here’s the critical exchange between Trump, Sanger, and the Times’s Maggie Haberman, which is worth reading in full:

SANGER: I was just in the Baltic States. They are very concerned obviously about this new Russian activism, they are seeing submarines off their coasts, they are seeing airplanes they haven’t seen since the Cold War coming, bombers doing test runs. If Russia came over the border into Estonia or Latvia, Lithuania, places that Americans don’t think about all that often, would you come to their immediate military aid? TRUMP: I don’t want to tell you what I’d do because I don’t want Putin to know what I’d do. I have a serious chance of becoming president and I’m not like Obama, that every time they send some troops into Iraq or anyplace else, he has a news conference to announce it. SANGER: They are NATO members, and we are treaty-obligated —— TRUMP: We have many NATO members that aren’t paying their bills. SANGER: That’s true, but we are treaty-obligated under NATO, forget the bills part. TRUMP: You can’t forget the bills. They have an obligation to make payments. Many NATO nations are not making payments, are not making what they’re supposed to make. That’s a big thing. You can’t say forget that. SANGER: My point here is, Can the members of NATO, including the new members in the Baltics, count on the United States to come to their military aid if they were attacked by Russia? And count on us fulfilling our obligations —— TRUMP: Have they fulfilled their obligations to us? If they fulfill their obligations to us, the answer is yes. HABERMAN: And if not? TRUMP: Well, I’m not saying if not. I’m saying, right now there are many countries that have not fulfilled their obligations to us.

In other words, Trump is saying that the US commitment to NATO hinges on whether particular NATO states — including the Baltics — have forked over enough cash.

Trump clearly doesn’t think of NATO in terms of an ironclad guarantee to allied states. He thinks of it as transactional, akin to a real estate deal or (less charitably) a protection racket: The United States only protects its weaker allies if they pay up.

Now, Trump’s comments aren’t coming out of nowhere. Members of the alliance are supposed to spend 2 percent of GDP of defense, to ensure that each country is bearing an equal burden. Only five out of 28 members hit that target, and it’s reasonable for Trump to ask them to do more.

But it’s one thing to tell allies to do more, and another thing to threaten to pull out of the alliance if they don’t. It’s the latter move, in particular, that’s so scary.

This threatens peace in Europe

Trump’s comments about NATO are like an arrow pointed straight at the heart of the alliance -- and, if implemented, would directly threaten the foundations of global peace itself.

Any president could simply choose not to abide by Article 5. But abrogating NATO agreements was always deemed unthinkable by both parties, which has played an important part in maintaining credible deterrence vis-à-vis Russia. Trump has put the idea of the US not defending NATO on the table, in a very real way.

This threatens the very integrity of NATO itself. If NATO allies start to think that the United States can’t be trusted to defend them, that NATO is just on paper, then they’ll start to wonder why they bother to adhere to this alliance in the first place. If Trump wins the election, this could cause them to exit the security agreement altogether.

According to the best available research, this would make war on the European continent far more likely.

One study, from professors Jesse C. Johnson and Brett Ashley Leeds, surveyed about 200 years of data on conflicts and concluded that "defensive alliances lower the probability of international conflict and are thus a good policy option for states seeking to maintain peace in the world."

Another study looked specifically at the period from 1950 to 2000 and found that "formal alliances with nuclear states appear to carry significant deterrence benefits." The US's formal agreements, then, deter aggression against its non-nuclear partners (like Germany and the Baltics).

In their new book on American grand strategy, Dartmouth scholars Steven Brooks and William Wohlforth also surveyed research from regional experts and found a similar consensus. In Europe, they write, "most assessments nonetheless sum up to the conclusion that NATO is a net security plus."

Trump, then, could end up weakening one of America’s most important security agreements — and may already have done so.

The nightmare scenario: US-Russia war

Trump’s comments are worse than just undermining NATO: By refusing to commit to the Baltics categorically, he encourages Russia to test American resolve in dangerous ways.

According to some Russia experts, Vladimir Putin’s ultimate wish in Europe is to break NATO. The way to do that, according to these scholars, is to expose the Article 5 guarantee as hollow: to show that when push comes to shove, the United States or other large NATO powers wouldn’t actually defend the weaker states.

The Baltic states would be the most likely scenario for this to happen. They are very small, they’re right on Russia’s borders, and they aren't really all that important to Western countries' own security. By threatening these states, Russia would force a question: Are the United States, Britain, and France really willing to sacrifice their own soldiers in defense of a tiny state?

In 2014, the Danish intelligence agency — note that Denmark is a NATO ally — publicly warned that this was a serious possibility:

Russia may attempt to test NATO’s cohesion by engaging in military intimidation of the Baltic countries, for instance with a threatening military build-up close to the borders of these countries and simultaneous attempts of political pressure, destabilization and possibly infiltration. Russia could launch such an intimidation campaign in connection with a serious crisis in the post-Soviet space or another international crisis in which Russia confronts the United States and NATO.

The critical issue in preventing this scenario, again, is the perception of NATO commitment. So long as Putin believes that the US and other major powers are firmly committed to the defense of their treaty allies, he’s unlikely to risk starting a war that he would almost certainly lose.

This is why Trump’s comments are so damaging: They send a direct signal to the Kremlin that a Trump-led America will be less than serious about the defense of NATO allies. This suggests that a ploy to break NATO might have a bigger risk of succeeding than previously thought.

But note that Trump also refused to say unequivocally that he wouldn’t abide by the NATO treaty. “I don’t want to tell you what I’d do because I don’t want Putin to know what I’d do,” he said.

But the entire point of NATO is that Putin needs to know what America will do. If he knows the US will defend the Baltics, then he will likely back off. If he knows the US won’t defend the Baltics, then we could have the breakup of NATO — which would be quite bad but wouldn’t immediately risk World War III.

The nightmare scenario, though, is that Putin’s confidence in NATO is undermined even though the United States under Trump remains committed to defending its treaty allies. That’s the scenario under which misperceptions potentially escalate into an actual war between the world’s two largest nuclear powers.

Max Fisher wrote an extended piece on how this uncertainty could plausibly escalate to war for Vox last year; I encourage you to read it. But the point, according the experts Fisher spoke to, is that a firm understanding that the US will defend its NATO allies is crucial.

"That kind of misperception situation is definitely possible, and that’s how wars start," Steve Saideman, a professor who studies NATO at Carleton University, told Fisher. He then scarily compared modern Europe with pre–World War I Europe: "The thing that makes war most thinkable is when other people don’t think it’s thinkable."

I’m not saying we’re all going to die now. We most likely aren’t. The risks of any sort of full-on war with Russia are already low, and might even move lower in a Trump presidency — he could simply cede Eastern Europe to the Kremlin). But we don’t have to worry about that just yet: The US hasn’t withdrawn from NATO, and Russia is still relatively unlikely to gamble on a lack of American resolve given that it would likely lose any conventional war with NATO powers.

Yet Russia’s calculus has shifted at least a little, and perhaps more so come January, making the risk of a catastrophic war a bit higher today than it was yesterday. Even if Russia isn’t emboldened to full-on test NATO, the consequences could be severe. Russia messing with Baltic countries could make many people’s lives far less secure, and risk more serious incidents in the process.

This is one of the main reasons that foreign policy experts were horrified by the notion of President Trump. Now that it’s no longer a hypothetical concern, Trump needs to walk back the uncertainty he generated.

He’s already tried with South Korea, another ally that he has threatened to pull American support from. On Thursday, Trump called South Korean President Park Geun-Hye and committed himself to protecting the US-Korean alliance. That’s a big deal, as South Korea has in the past contemplated acquiring nuclear weapons to protect itself from North Korea (Trump once suggested that wouldn’t be such a terrible idea).

Trump needs to do something similar with NATO allies, telling the members of the alliance publicly that his administration will defend NATO allies and respect Article 5 without reservations.

This will be one of the first tests how an America with Trump’s finger on the nuclear trigger will work. We’re about to find out if he passes.

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