Already locked in a heated impeachment battle, President Trump found a new way to rile Washington this week. Late Sunday, the White House announced that it would not impede an imminent Turkish invasion into northeastern Syria. By Monday morning, my colleagues confirmed that the United States had begun withdrawing troops from the Syrian-Turkish border, where they had been stationed alongside U.S.-allied Syrian Kurdish fighters. It seemed the first phase of a possibly new and destabilizing military conflagration in war-ravaged Syria. By Monday evening, there were already reports of Turkish artillery shelling and airstrikes on Syrian Kurdish positions.

The moves followed a Sunday phone call between Trump and Turkish President Recep Tayyip Erdogan, whose government considers the main Syrian Kurdish faction that fought the Islamic State to be as much of a danger as the Islamist militants themselves. That’s because of their direct ties to the Kurdistan Workers’ Party, or PKK, an outlawed separatist group in Turkey that’s designated as a terrorist organization by Washington and Ankara.

Erdogan and his government long resented the outsize influence of the Syrian Democratic Forces, the name of the coalition of Syrian Kurdish-led militias that was instrumental in rolling back the Islamic State, as well as the outside support they received from the United States and other European partners. No matter the thousands of lives lost by the SDF, Trump now appears to have given Erdogan what he’s wanted — an open invitation to weaken if not break Rojava (what Syrian Kurds call an enclave in northeastern Syria largely administered by the SDF) and potentially repopulate a belt of territory with Syrian refugees currently living in Turkey.

A definitive U.S. withdrawal from northern Syria — or whatever it is that Trump championed in tweets Monday morning calling for an end to America’s “endless wars” — could have serious ramifications. If Turkey pursues a significant ground invasion, it would overrun SDF defenses, fracture their fighting forces, compel a possible exodus of Syrian Kurdish refugees toward Iraq (a country that’s hardly stable itself), and, as many security analysts fear, create enough of a security vacuum to pave the way for the Islamic State’s resurgence.

For that reason, many Trump allies expressed outrage with his decision-making. Nikki Haley, Trump’s former ambassador to the United Nations, tweeted that leaving the Syrian Kurds “to die is a big mistake.” Sen. Lindsey O. Graham (R-S.C.), who has carried water for Trump on almost all other fronts, warned that a Turkish foray into Syria would provoke sanctions from Congress and a move to suspend Ankara’s participation in NATO. Senate Majority Leader Mitch McConnell (R-Ky.) told reporters that “a precipitous withdrawal of U.S. forces from Syria would only benefit Russia, Iran” and the Syrian regime.

Trump predictably justified his actions not with a strategic vision for a troubled region, but campaign talking points. He told reporters on Monday afternoon that he was following through on what he “got elected on” — in this instance, disentangling the United States from the Middle East’s intractable conflicts. He said it was the responsibility of other countries, including Turkey, to deal with what’s left behind. For good measure, he threatened Ankara with crippling economic sanctions if the Turks did anything “outside of what we think is humane.”

Whatever Trump thinks “inhumane” may mean, it was a far cry from the somewhat rambling remarks Trump made at a news conference a year ago hailing the United States’ Kurdish partners.

“We do get along great with the Kurds. We’re trying to help them a lot,” the president said then in response to a Kurdish journalist’s question about U.S. interests in Syria and Iraq after the defeat of the Islamic State. “Don’t forget, that’s their territory. We have to help them. … They fought with us. They died with us. They died. We lost tens of thousands of Kurds, died fighting ISIS. They died for us and with us. And for themselves. They died for themselves. They’re great people. And we have not forgotten. We don’t forget.”

The Middle East’s largest stateless ethnic group, close to 40 million Kurds are scattered throughout Turkey and parts of Iran, Iraq and Syria. They are hardly a monolith; numerous political and factional divisions exist within Kurdish communities. There’s little love lost, for example, between the party that controls the semiautonomous regional government in Iraqi Kurdistan and the PKK-linked elements across the border in Syria. But they all share a long history of frustrated national ambitions, punitive state violence and abandonment on the world stage.

Betraying the Kurds is, at this point, an American tradition. In the early 1970s, as a favor to the shah of Iran, the administration of President Richard M. Nixon helped foment a Kurdish uprising in Iraq. But once Baghdad and Tehran mended fences in 1975, the United States turned its back on the insurrection, many of whose combatants were slaughtered or driven into exile. Henry Kissinger, Nixon’s secretary of state, is said to have quipped about Kurdish disappointment then that “covert action should not be confused with missionary work.” In 1991, President George H.W. Bush called on Iraqis to rise up against the dictatorship of Saddam Hussein. But the United States then stood by as Iraqi forces brutally crushed separate Kurdish and Iraqi Shiite rebellions.

We’ll see what script from history gets reread in the coming weeks. In early August, the United States and Turkey agreed to a “security mechanism” that led to Syrian Kurdish forces pulling back from near the Turkish border and dismantling some of their critical fortifications. The SDF cooperated in good faith but now is even more vulnerable to a Turkish advance. A statement from the SDF on Monday expressed disappointment that U.S. forces “did not carry out responsibilities” and a grim resolve “to defend our land at all costs.”

What distinguishes the current moment of Kurdish betrayal from earlier eras is perhaps its seeming strategic incoherence. On Monday morning, officials in the Pentagon and State Department voiced bewilderment over the president’s tweeted announcements; reports suggested that neither the SDF nor other U.S. coalition partners on the ground were given warning of Trump’s decision.

Brett McGurk, Trump’s former top diplomat in the war against the Islamic State, savaged his ex-boss on Twitter. “The value of an American handshake is depreciating,” he said. “Trump today said we could ‘crush ISIS again’ if it regenerated. With who? What allies would sign up? Who would fight on his assurances?”