The story of the past few days in news has become clear: It was America versus its allies.

After President Trump’s meeting with Kim Jong Un, Trump announced that he would be suspending joint military exercises — “the war games,” as he put it — with South Korea, as a gesture of goodwill toward the North. This seems to have come as a shock to America’s allies in Seoul: South Korean President Moon Jae-in’s office released a statement saying “we need to find out the precise meaning or intentions of President Trump’s remarks,” implying they had no idea this was coming.

The weekend before, at the G7 summit, a confab for leaders of seven wealthy democracies, Trump got into a fight with Canadian Prime Minister Justin Trudeau over trade. When Trudeau criticized Trump’s imposition of new tariffs on steel and aluminum imports — saying “Canadians did not take it lightly” — Trump called him “very dishonest and weak” on Twitter. Peter Navarro, one of his top trade advisers, said on Fox News that there was “a special place in hell” for Trudeau.

These two incidents aren’t the only times Trump has infuriated American allies in the past year. Just last month, he pulled the United States out of the Iran nuclear deal — a painstakingly negotiated agreement involving several of America’s top European allies. Last June, he withdrew from the Paris climate agreement. And this all came amid constant carping about how America’s NATO allies needed to pay their “fair share,” and after Trump’s past musings about how he might not defend allies if they didn’t.

Trump’s betrayal of South Korea and eruption at Trudeau are not one-offs, or events you can write off as simple quirks of the president’s personality. It is part of a broader slate of Trump policies and diplomatic efforts that have, put together, fundamentally weakened America’s ties with its traditional allies — in ways that could have potentially disastrous consequences for the world.

America’s alliances depend on the US’s reputation for upholding its agreements and treating its allies fairly. Trump’s blithe disregard for diplomacy and international agreements has damaged the US’s reputation in a way that some scholars worry may be irreparable. And a deep body of research on international relations suggests that the strength of America’s alliances in Europe and East Asia have played a pivotal role in preventing another world war. The more Trump mucks around with American alliances, the more unstable the world becomes — making a large military conflict more imaginable.

Such a disaster hopefully will never happen. Indeed, it’s nearly impossible for most of us to imagine one happening now: We live in one of the most peaceful times in human history, with some of the lowest rates of deaths in conflict ever recorded.

But that’s precisely the point: Our age is such an anomaly when it comes to conflict that we aren’t entirely sure what could trigger a return to the violent historical norm. Serious, lasting damage to the American-led alliance system might do the trick.

And President Trump’s foreign policy could well be doing such damage. His approach is so erratic, so contemptuous of America’s traditional way of doing business, that US allies are openly worrying in a way that we haven’t seen in modern history. This affects world politics at their most fundamental level, undermining an otherwise stable global system in ways that we are only dimly capable of perceiving.

The past week’s news was a particularly naked demonstration of what had been, to date, one of the most subtle and insidious effects of the Trump presidency: an erosion of the foundations of the political system that defines — and protects — the modern world.

Trump is attacking the heart of American alliances: trust

In 1956, a leaked Defense Department memo proposing the withdrawal of US troops from Europe terrified European allies; German Chancellor Konrad Adenauer declared privately that “NATO is finished.” The Nixon administration’s diplomatic outreach to China in 1972 led both Japanese and South Korean policymakers to fear that the US would cut ties to curry favor with Beijing. Seoul even began a covert nuclear weapons program to defend itself in the event of American abandonment.

These examples illustrate a common historical pattern: Alliances between a stronger state and weaker partners become at risk of collapse when the weaker state no longer feels like it can trust the stronger state.

“An alliance may dissolve if its members begin to question whether their partners are genuinely committed to providing assistance,” Harvard scholar Stephen Walt writes in a 1997 survey of the historical record. “This problem will be more severe when ... there is a large asymmetry of power among the member-states.”

The US has prevented allies from truly losing faith in the past basically by reassuring them. Presidents promise allies that they’d never abandon them, and provide them tangible goods — like US troop deployments to their country, military assistance, or even trade agreements — to demonstrate America’s continuing commitment.

America’s history of managing allies speaks to a fundamental truth about these agreements: They are grounded entirely in trust. Ultimately, an alliance is nothing but a promise: that the United States will defend its allies, either in Europe or East Asia, in the event of an unspecified future attack. There’s nothing an ally can do to force the United States into defending them; they just have to take America’s word for it.

As a result, a country’s reputation for treating its allies well is crucially important to determining whether its alliances can work. A 2008 paper by Douglas Gibler, a professor at the University of Alabama, found that states that did not honor their commitment to allies in the past were considerably less likely to forge new alliances in the future.

Trade pacts, environmental agreements, and the Iran nuclear deal don’t touch the core US promise in US military alliances — to defend allies in the event of an attack. But backing out of such accords does serious damage to Trump’s reputation as a trustworthy ally. Withdrawing the United States from major agreements and imposing tariffs on allies, all while cozying up to Vladimir Putin and sitting down with Kim Jong Un, tells US allies that Trump doesn’t feel particularly bound by formal agreements or the traditional thrust of US foreign policy. If he decides that an agreement doesn’t put “America First,” he’s perfectly willing to kick it to the curb.

Trump has openly said this in the past. In July 2016, he told the New York Times that he would be willing to disregard Article 5, the provision of NATO’s founding treaty committing allies to defending each other in the event of the attack.

“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 the Times.

Trump has tried to walk back that kind of rhetoric as president. But his actions in the past year — especially the snap cancellation of longstanding military exercises with South Korea — make the threat of abandonment seem all too credible to allies. When NPR asked Tomas Valasek, the recently retired Slovak ambassador to NATO, if Trump would defend his country or another ally in the event of an attack, he couldn’t bring himself to say yes.

“The honest answer is none of us quite knows,” Valasek said. “His heart is not into alliance. He has a zero-sum view of the world. He believes in no permanent friendships, no permanent allies. You know, that’s not the sort of mindset that prepares him well for sort of standing by the side of an ally in case of a crisis.”

The result of Trump’s reputation for unreliability, then, is a weakening of American alliances. Allies will trust the United States less, and may start looking for alternatives to depending so heavily on the United States. Enemies will see cracks in US alliances and may attempt to exploit them.

“The liberal international order depends on us believing that agreements like treaties [and] international organizations have long-term staying power beyond leadership change,” Brett Ashley Leeds, a scholar of US alliances at Rice University, tells me. “The scariest part is the fact that [Trump] is creating so much uncertainty about what US policy is going to be.”

How the weakening of American alliances could lead to a massive war

There has never, in human history, been an era as peaceful as our own. This is a hard truth to appreciate, given the horrible violence ongoing in places like Syria, Yemen, and Myanmar, yet the evidence is quite clear.

Take a look at this chart from the University of Oxford’s Max Roser. It tracks the number of years in a given time period in which “great powers” — meaning the militarily and economically powerful countries at that time — were at war with each other over the course of the past 500 years. The decline is unmistakable:

This data should give you some appreciation for how unique, and potentially precarious, our historical moment is. For more than 200 years, from 1500 to about 1750, major European powers like Britain and France and Spain were warring constantly. The frequency of conflict declined in the 19th and 20th centuries, but the wars that did break out — the Napoleonic conflicts, both world wars — were particularly devastating.

The past 70 years without great power war, a period scholars term “the Long Peace,” is one of history’s most wonderful anomalies. The question then becomes: Why did it happen? And could Trump mucking around with a pillar of the global order, American alliances, put it in jeopardy?

The answer to the second question, ominously, appears to be yes. There is significant evidence that strong American alliances — most notably the NATO alliance and US agreements to defend Japan and South Korea — have been instrumental in putting an end to great power war.

“As this alliance system spreads and expands, it correlates with this dramatic decline, this unprecedented drop, in warfare,” says Michael Beckley, a professor of international relations at Tufts University. “It’s a really, really strong correlation.”

A 2010 study by Rice’s Leeds and the University of Kentucky’s Jesse C. Johnson surveyed a large data set on alliances between 1816 and 2000. They found that countries in defensive alliances were 20 percent less likely to be involved in a conflict, on average, than countries that weren’t. This holds true even after you control for other factors that would affect the likelihood of war, like whether a country is a democracy or whether it has an ongoing dispute with a powerful neighbor.

In a follow-up paper, Leeds and Johnson looked at the same data set to see whether certain kinds of alliances were more effective at protecting its members than others. Their conclusion is that alliances deter war best when their members are militarily powerful and when enemies take seriously the allies’ promise to fight together in the event of an attack. The core US alliances — NATO, Japan, and South Korea — fit these descriptors neatly.

A third study finds evidence that alliances allow allies to restrain each other from going to war. Let’s say Canada wants to get involved in a conflict somewhere. Typically, it would discuss its plans with the United States first — and if America thinks it’s a bad idea, Canada might well listen to them. There’s strong statistical evidence that countries don’t even try to start some conflicts out of fear that an ally would disapprove.

These three findings all suggest that NATO and America’s East Asian alliances very likely are playing a major role in preserving the Long Peace — which is why Trump’s habit of messing around with alliances is so dangerous.

According to many Russia experts, Vladimir Putin’s deepest geostrategic goal is “breaking” NATO. The member states where anyone would expect him to test NATO’s commitment would be the Baltics — Estonia, Latvia, and Lithuania — small former Soviet republics that recently became NATO members.

We can’t predict if and when a rival like Putin would conclude that America’s alliances seemed weak enough to try testing them. Hopefully, it never happens.

But the more Trump attacks the foundations of America’s allies, the more likely things are to change. The absolute risk of a Russian invasion of a NATO state or a North Korean attack on the South is relatively low, but the consequences are so potentially catastrophic — nuclear war! — that it’s worth taking anything that increases the odds of such a conflict seriously.

The crack-up of the West?

The world order is a little like a game of Jenga. In the game, there are lots of small blocks that interlock to form a stable tower. Each player has to remove a block without toppling the tower. But each time you take out a block, the whole thing gets a bit less stable. Take out enough blocks and it will collapse.

The international order works in kind of the same way. There are lots of different interlocking parts — the spread of democracy, American alliances, nuclear deterrence, and the like — that work together to keep the global peace. But take out one block and the other ones might not be strong enough to keep things together on their own.

At the end of the Cold War, British and French leaders worried that the passing of the old order might prove destabilizing. In a January 1990 meeting, French President François Mitterrand told British Prime Minister Margaret Thatcher that he feared a united Germany could seize control of even more territory than Hitler. Some experts feared that in the absence of the external Soviet threat, Western European powers might go back to waging war with each other.

Thankfully, those predictions turned out to be wrong. There are multiple reasons for that, but one big one — one that also helped keep relations between other historical enemies, like South Korea and Japan, peaceful — is a shared participation in US alliance networks. The US serves as the ultimate security blanket, preventing these countries from having to build up their own armaments and thus risk a replay of World War I. But if American alliance commitments become and remain less credible, it’s possible this order could crack up.

America’s partners aren’t stupid. They understand that Trump is the product of deep forces in American politics, and that his victory might not be a one-off. If they think that this won’t be the last “America First” president in modern history, depending on America the way that they have in the past could quickly become a nightmare.

The worst-case scenarios for a collapse in the US alliance system are terrible. Imagine full Japanese and German rearmament, alongside rapid-fire proliferation of nuclear weapons. Imagine a crack-up of NATO, with European powers at loggerheads while Russia gobbles up the Baltic states and the rest of Ukraine. Imagine South Korea’s historical tensions with Japan reigniting, and a war between those two countries or any combination of them and China.

All of this seems impossible to imagine now, almost absurd. And indeed, in the short run, it is. There is no risk — zero — of American allies turning on each other in the foreseeable future. And it’s possible that the next president after Trump could reassure American allies that nothing like this could ever happen again.

But the truth is that there’s just no way to know. When a fundamental force for world peace starts to weaken, no one can really be sure how well the system will hold up. Nothing like this — the leader of the world’s hegemon rounding on its most important allies — has ever happened before.

What Donald Trump’s presidency has done, in effect, is start up another geopolitical Jenga game. Slowly but surely, he’s removing the blocks that undergird global security. It’s possible the global order survives Trump — but it’s just too early for us to say for sure.

Given the stakes, it’s a game we’d rather not play.