A little over a month ago, White House officials suddenly realized that tensions between South Korea and Japan—the two top U.S. allies in North Asia—were spiraling out of control. Like most Americans who don’t follow Asian politics, they seemed unaware, or unfazed, that these tensions had erupted periodically for the past 70 years—and that the eruptions have usually been quelled by American mediation.

The issues between the two countries have been the same all along. What’s different, this time around, is that President Donald Trump—unlike all previous presidents—has had, until very recently, no interest in stepping in.

“How many things do I have to get involved in?” he asked last month, in a tone of exasperation, when he first heard requests to help quell the tensions. The implication was that he didn’t much want to get involved in this one. And so the tensions spiraled.

Those issues between the two countries amount to a toxic brew of nationalist resentments dating back more than a century. They ought to be a cautionary tale in this era when our own country is repackaging old tribal disputes into seething partisan politics.

The background is this: In 1910, Japan colonized South Korea, recruited laborers at abysmally low wages, and, during World War II, forced South Korean women to serve as “comfort women”—essentially sex slaves—to Japanese servicemen. The memory of this history—still potent, though few of its victims are still alive—is the source of the tensions today.

For a while, it seemed that this tawdry chapter had been closed. A treaty signed in 1965 obligated the Japanese government to reimburse the underpaid workers (many of whom were still alive at the time). Another treaty, signed in 2015, compensated the surviving comfort women and many families of those who were deceased.

“How many things do I have to get involved in?” — Donald Trump

Top-tier U.S. officials—including, in the more recent treaty, President Barack Obama—were deeply involved in the negotiations of both treaties.

Gradually, over the past two years, both treaties have fallen apart—and no U.S. official, certainly not Donald Trump, has intervened to hold them together.

Many South Korean citizens protested the limited terms of the “comfort women” agreement upon its signing, and when President Moon Jae-in took office in 2017, he shut down the foundation that had been distributing the Japanese funds.

Then, last year, a lawsuit was filed in South Korean court on behalf of the underpaid laborers, seeking reimbursement from Japanese companies that had participated in the occupation. The 1965 treaty—which also established diplomatic relations between South Korea and Japan—had explicitly placed liability strictly on the Japanese government. It also set up an arbitration board—consisting of one person each from Japan, South Korea, and a third party—to hear and resolve any complaints. The lawyers for the laborers argued that individuals should have the right to sue for damages from the offending companies. (Under the treaty, Japan paid the South Korean government, which, rather than distributing the money to the underpaid laborers, poured it into industrial investment, which spurred enormous economic growth.) The judge sided with the laborers. In a split decision, which took everyone by surprise, the South Korean Supreme Court upheld that verdict.

That was the pivot. The Japanese companies, backed by the Tokyo government, refused to comply. The Seoul government then threatened to seize Japanese assets in South Korea. Last month, Japan restricted exports of materials critical to South Korea’s semiconductor and flat-panel industries, while also removing the country from its “whitelist” of trusted trading partners. South Korea removed Japan from its own similar list.

Finally, just this week, South Korea canceled a treaty with Japan that was signed in 2016 (again, under heavy prodding from the Obama administration), under which the two agreed to share top-secret intelligence information with each other.

“This is the most serious rupture,” says Daniel Sneider, lecturer at Stanford University, who was written extensively about these growing tensions and their roots. Not only did the 2016 treaty allow rapid transmission of intelligence about North Korea’s nuclear sites, missile tests, and other possible threats; it also led to extensive contacts between South Korean and Japanese military commands—contacts that were kept out of public view and that might now be terminated along with the treaty.

The widening gulf between the two allies will also make it harder to devise a common position on negotiating with North Korea, assuming that, at some point, a U.S. president is interested in negotiating, rather than letting Kim Jong-un do whatever he wants as long as he stops short of test-launching long-range missiles.

Ever since its creation, just after World War II, North Korea—always a small, impoverished nation—has followed a strategy of playing off the divisions among its larger neighbors. The widening gulf between South Korea and Japan gives Kim much more room to play.

The Trump administration woke up to the dangers just last month, after the mutual export bans and South Korea’s subsequent threat to seize Japanese assets. Matt Pottinger, the senior Asia specialist on the National Security Council, arranged a trip to the two countries. But John Bolton, Trump’s national security adviser (and Pottinger’s boss), headlined the visit—and, while he mentioned the tensions between the two countries, he spent much more time trying to get their leaders to join the U.S.-led coalition against Iran. (He failed.)

Trump had already said he didn’t want to get involved in a trade dispute between South Korea and Japan. So both of them felt free to ratchet up the tensions—which are driven by domestic politics much more than any national security considerations.

“Obviously, Trump didn’t create this situation,” Sneider told me. “But what’s happening is a consequence of the vacuum of leadership in Washington—and that is created by the Trump administration.”

Scott Snyder, director of the Program on U.S.-Korea Policy at the Council on Foreign Relations, agrees. “The Obama administration was pretty deep behind the scenes in facilitating improvement of Japan–South Korea relations in 2015–16,” he says. As part of his effort to “pivot” toward Asia, Obama created a quarterly summit among the deputy foreign secretaries of the United States, South Korea, and Japan. As in most previous administrations, U.S. ambassadors and assistant secretaries in charge of Asian affairs at the Defense and State departments spoke to their counterparts almost daily, putting out small fires before they erupted into four-alarm blazes. All of this ended under Trump. Ambassadors and assistant secretaries for the region weren’t even appointed until very recently.

Some U.S. officials have tried to put out the flames since this summer’s spurt of tensions. Snyder notes that U.S. emissaries proposed a “standstill” agreement between the two governments, but Moon and Japanese Prime Minister Shinzō Abe “shrugged it off” and paid no consequence for doing so: a clear signal that the United States would no longer use its leverage to exert compromises. Earlier this month, Trump finally said he was “concerned” that the two allies were “not getting along with each other.” But he proposed no agenda or demands, so nothing is likely to happen.

Trump’s main misperception, even now, is seeing the rising tensions as a mere trade dispute. “It’s not a trade dispute,” Daniel Sneider says. “Trade is the main arena in which these larger issues”—issues of nationalism, bitterness, honor, and history—“play out.”

Obama didn’t fully grasp the larger dimensions, either. Sneider recalls him telling the Japanese and South Korean leaders, “You have to look forward,” as if history was something that merely happened long ago rather than a force that pervades daily life and shapes streams of consciousness several decades after the slights and furies began to accumulate.

This is a common failing of Americans, especially when we gaze at disputes agitated not merely by decades but by centuries of animus—sectarian wars in the Middle East, border wars in South Asia, tribal wars in Africa, civil wars everywhere—and wonder why the combatants can’t just move on.

It’s a strange blindness, since we Americans still very much carry the wounds of our past, notably the sin of slavery, which continues to animate our politics, culture, economics, and society. The myth of the melting pot exerts such a strong force that, in many corners of life, it’s come true—and where it hasn’t, it has allowed many of us to pretend that it has. (Look at the preposterous backlash to the New York Times’ recent 1619 Project, revealing that many conservatives deny that slavery has exerted any legacy on modern life whatsoever.) Maybe that’s why we have avoided a resurgence of civil wars. Maybe that avoidance, brought on by evasion, lies at the heart of “American exceptionalism”—though now we too are wondering how long that can sustain itself.

Many other countries face the force of history all the time. South Korea and Japan are unusual in that they’re divided by this force yet at the same time are both our allies. The challenge, which previous presidents have taken up to some degree, is to recognize this fact, to confront its complexities, to help the countries contain the natural tendency for conflict: in short, to act like the leader of an alliance, not just for their benefit but for our own.