Two of America’s closest allies in the Middle East went to war on Wednesday—and Donald Trump didn’t seem to care. In what may have been the first declaration of hostilities on Twitter, the Turkish President, Recep Tayyip Erdoğan, announced that Turkey, a NATO ally, had launched an invasion of Syria, to clear out a Kurdish-led militia that controls about a third of the country. The militia, known as the Syrian Democratic Forces, has been allied with the United States for the past five years in the war against ISIS. Both sides have been equipped by the United States, albeit in vastly different ways.

Erdoğan dubbed the invasion Operation Peace Spring. It is anything but. Panic swept across northern Syria as Turkey’s warplanes pounded Kurdish towns and artillery fired across the border, in order to—in ironic military jargon—“soften up” the terrain for a ground offensive. The S.D.F. posted videos on social media of the aftermath, showing fires, destruction, and bodies on the ground.

The region’s latest war is a deeply uneven match—and has the potential to be a slaughter of the Kurds. Turkey, which contributes the second-largest force to NATO, is considered the ninth most powerful military in the world. It has more than three hundred and fifty thousand active-duty soldiers—and twice that with reserves. The S.D.F. militia has about sixty-thousand men and women in uniform, including reserves; they have only rudimentary training.

Turkey also has some of the world’s most advanced weaponry—much of it American or other Western matériel—including some five hundred warplanes, more than ten thousand tanks and armored vehicles, and thousands of artillery pieces. The S.D.F. is lightly armed, with vintage rifles and machine guns, a few (mostly captured Soviet-era) tanks, and no air force. (The U.S.-led coalition provided airpower during the war against ISIS.) Its best weapons are mortars. The United States provided the S.D.F. with night-vision goggles, body armor, and armored vehicles for transport.

As Turkey unleashed its military, the Pentagon released a statement underscoring the importance of its Kurdish allies in the ongoing campaign against the Islamic State. The S.D.F. forced ISIS out of its last bastion in Syria in March. But the jihadi movement is estimated to have between twenty thousand and thirty thousand fighters still hiding in the deserts, caves, and mountains in northeastern Syria and Iraq. Since March, it has launched sporadic attacks, including car bombings, in both countries. As if on cue, ISIS fighters reportedly attacked an S.D.F. position in Raqqa—the former Islamic State capital—on Wednesday, as Turkey was attacking the S.D.F. further north.

The Syrian Democratic Forces “are leading the way to find and eliminate ISIS fighters, finance, logistics, and media networks,” U.S. Central Command reported on Wednesday. In just three days over the past week, S.D.F. commandos captured a top ISIS financier, who is linked to a car bombing (and was in possession of explosives); an explosives expert; and a commander of an ISIS sleeper cell. Removing ISIS fighters, weapons, and explosives “remains a top priority” as the jihadi movement “continues to plot attacks against innocent civilians and our partners throughout Iraq and northeast Syria,” the Pentagon said. By the end of the day, however, the U.S. had ordered a halt both to operations against ISIS and to further U.S. training of the S.D.F. forces who were stabilizing the area so that the last of the American presence could leave. The S.D.F. was unable to fight on both fronts at the same time.

On Twitter, Erdoğan said that Turkey’s goal is to “prevent the creation of a terror corridor across our southern border.” The Kurdish contingent in the S.D.F. has ties to Kurdish separatists who have sought autonomy in Turkey. Kurds are the largest minority in Turkey—and the largest minority in the world without a state. They are spread across Turkey, Syria, Iraq, and Iran. They were promised a state in the reconfiguration of the Middle East after the First World War, but the agreement was later abandoned. Erdoğan considers the Kurds to be a terrorist group. Turkey wants to create a buffer “safe zone” inside of Syria along the two countries’ five-hundred-and-seventy-mile border.

The United States is the one country with sufficient leverage—as the leading nation in NATO and the head of the coalition backing the S.D.F.—to have negotiated a way out of war. Trump has instead commended both sides, but actually done little. “We may be in the process of leaving Syria, but in no way have we Abandoned the Kurds, who are special people and wonderful fighters,” he tweeted, on Tuesday. “Likewise our relationship with Turkey, a NATO and Trading partner, has been very good.”

After Erdoğan’s tweet, Trump called the invasion a “bad idea.” But he had effectively green-lighted Turkey’s offensive by pulling U.S. troops from the border after Erdoğan had shared his plans with him, in a telephone call, on Sunday. “Turkey,” the President added, in a statement, “has committed to protecting civilians, protecting religious minorities, including Christians, and ensuring no humanitarian crisis takes place—and we will hold them to this commitment.”

Trump’s words did little to quell the furor among even his closest allies in Washington. “Pray for our Kurdish allies who have been shamelessly abandoned by the Trump Administration,” Senator Lindsey Graham, of South Carolina, tweeted. “This move ensures the reemergence of ISIS.” The Wyoming congresswoman Liz Cheney also unleashed on the President: “News from Syria is sickening. Turkish troops preparing to invade Syria from the north, Russian-backed forces from the south, ISIS fighters attacking Raqqa. Impossible to understand why @realDonaldTrump is leaving America’s allies to be slaughtered and enabling the return of ISIS.”

Trump shot back, in a Twitter rant, that his critics did not understand the “BIG PICTURE” in the world’s most volatile region. “GOING INTO THE MIDDLE EAST IS THE WORST DECISION EVER MADE IN THE HISTORY OF OUR COUNTRY!” he tweeted.

Until December, the United States had two thousand troops in Syria to coördinate with and support the S.D.F. This year, the number is estimated to have been cut in half after Trump announced plans for withdrawal. This week, he ordered small teams of American Special Forces based in two towns along the border with Syria—Tal Abyad and Ras al Ain—to move. Turkey’s ground offensive entered at four points near those two Kurdish towns, local media reported.

The invasion has profound implications for the entire region. Syria, which is twice the size of Portugal, or about the size of North Dakota, has already been dismembered in two wars. An uprising against President Bashar al-Assad, in 2011, disintegrated into a civil war. The northern province of Idlib is still controlled by rebels. In 2014, ISIS seized a third of the country for its caliphate. After the Islamic State collapsed, the S.D.F. took control of the northeast. Any new Turkish buffer zone would further complicate efforts to maintain Syria’s territorial borders, which were carved out a century ago, by Europeans, after the First World War.