This piece has been updated.

The commander of Syrian Kurdish forces didn’t use the word “betrayal.” But with U.S. troops withdrawing from the Turkish-Syrian border and Turkish troops poised to attack, he warned on Monday that a bloodbath could be ahead — and pleaded for continued U.S. support.

Gen. Mazloum Abdi, the commander of the Kurdish-led militia known as the Syrian Democratic Forces, said during a telephone interview from his headquarters inside Syria that Turkish troops “are preparing to cross the border” and that a major assault could come in “a matter of days.”

The Kurdish commander’s pleas that the United States not abandon an ally struck a chord in a Washington that is whipsawed by President Trump’s increasing practice of diplomacy by impulse and whim. Even Senate Republicans who usually acquiesce to Trump’s policies were vocal in protesting Trump’s move to abandon our Syrian Kurdish allies, who did most of the fighting and dying against the Islamic State.

This latest chapter of the Syrian horror story followed a White House announcement late Sunday that Trump was, in effect, leaving the border unprotected in the face of a possible Turkish attack, despite repeated U.S. pledges over the past three months that the United States would seek to forestall such an assault if the Kurds pulled back troops and heavy weapons from the border zone.

Mazloum told me that U.S. forces had withdrawn from observation posts along the frontier but remain in major SDF garrisons, such as Kobane. A knowledgeable U.S. source said that about 1,000 U.S. troops are now in the country and that the goal of the withdrawal was “getting out of the way” of any Turkish attack, rather than quitting Syria entirely and immediately.

Mazloum said he was sending “thousands” from his total force, which he estimated at 70,000, north toward the border to defend Kurdish areas. He said that despite Turkey’s superior firepower, “it will be a big battle,” with the risk of significant bloodshed.

The Syrian Kurdish commander said the real danger was that Turkish forces would attempt “ethnic cleansing” — evicting Kurds from their ancestral lands in northeast Syria and installing Syrian Arab refugees who have been living in camps in Turkey since the Syrian civil war began.

“If ethnic cleansing happens in our area, or they kill Kurds and bring Arabs in, this will be the U.S.’s responsibility,” Mazloum said, speaking through a translator. At another point, the normally unemotional Mazloum spoke of the danger of “genocide” against the Kurds.

The U.S. retreat under Turkish threats of invasion shocks the Kurds (and U.S. members of Congress) in part because the SDF was such a stalwart ally of the United States in the campaign to destroy the Islamic State. Mazloum told me in an interview in Kobane in July that his forces had suffered 11,000 killed and 24,000 wounded in fighting the Islamic State since 2014. U.S. deaths in that campaign were fewer than 10.

“We are asking President Trump to keep his promise” to preserve safety and security for Kurdish allies, Mazloum said. He asked U.S. policymakers “to stop this decision and stop the Turkish attack.” He said that abandoning the Kurds “is hurting U.S. interests and reputation. It’s not acting according to American principles.”

One devastating consequence of a Turkish invasion is that it might allow hardened Islamic State fighters now in prisons controlled by the SDF to escape and resume their terrorist attacks. Mazloum told me in July that these prisons are holding 2,500 foreign fighters, including about 1,000 Europeans, in addition to 3,000 Iraqis. “There is a possibility that because of lack of security, they will be out of control, and escape,” Mazloum warned Monday.

Mazloum scoffed at Sunday’s White House statement that the Islamic State prison camps would be protected by Turkish forces, noting that the foreign fighters had initially entered Syria through a porous Turkish border.

I’ve talked with Mazloum a half- ­dozen times over the past few years, sometimes in person amid the sun- ­beaten fields of northeastern Syria. He’s a hard man in a hard country, but he decided to trust the United States five years ago, as we began the war against the Islamic State. Now, as his commanders complain bitterly about the unreliability of U.S. promises, he must wonder why he was so accommodating.

The United States has had other moments when it abandoned allies for domestic political reasons. But history is likely to record Trump’s apparent decision to sacrifice the Kurds as a particularly egregious example of presidential disdain for the moral consequences of foreign policy choices.

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