It will be a crowded field when the West’s greatest geopolitical betrayals are measured against each other, but surely President Trump’s turnabout on the Kurdish people of northern Syria represents a standout. In a phone call last Sunday night the president abruptly reversed U.S. policy, against the counsel of both the Pentagon and the State Department, abandoning the Kurds—again—to the brutality of a more powerful neighbor.

Mr. Trump had called Recep Tayyip Erdogan to press the Turkish president on his recent decision to purchase Russian warplanes. But by the time the call had ended, instead of thwarting the weapons buy, Mr. Trump had been persuaded to withdraw the small contingent of U.S. troops that had been based in northern Syria with U.S. regional allies, the Kurdish-led Syrian Democratic Forces. The U.S. presence had been the S.D.F.’s sole protection.

It is hard to overstate how strategically ruinous the president’s volte-face on the S.D.F. will likely prove to be.

Since 2015, this alliance of Kurdish People’s Protection Units, Free Syrian Army elements and Assyrian and other militias had been the tip of the U.S. spear against ISIS extremism. The Kurds suffered thousands of battlefield losses during that fight but had been encouraged to believe that their sacrifice was sufficient to earn at least a seat at the table when the final resolution of the Syrian catastrophe is negotiated. Now both the S.D.F. and the United States will likely be on the sidelines when Syria’s President Assad, Iran, Russia and Turkey eventually get around to concluding the mind-numbing violence in Syria.

Mr. Erdogan justifies the offensive as a strike against potential terrorist elements he perceives within the S.D.F., even as his army and its allied Sunni militia rain actual terror down on border communities, including a number of Christian villages. This latest Turkish offensive will propel the same ethnic cleansing by forced relocation that followed the Afrin campaign in 2018, when as many as 300,000 people were displaced. Indeed, hundreds of Kurdish, Syriac Christian and Yazidi families are already choking roadways northward, attempting to escape the indiscriminate Turkish bombardment of border communities.

It is hard to overstate how strategically ruinous the president’s volte-face on the S.D.F. will likely prove to be. The small U.S. deployment not only protected S.D.F. gains; it ensured that the United States remained a player in Syria and allowed U.S. strategists a continuing role in the containment of the ISIS malignancy. Turkey apparently offered the president nothing in return for his capricious abandonment of the nation’s hard-won achievements in Syria. Now NATO has fractured as European leaders rush to condemn the Turkish attack, and thousands of captured ISIS militants are likely to escape into the wind as S.D.F. troops are forced to abandon detention duty for the frontline.

The Kurds will have no choice but to seek a rapprochement with Mr. Assad if they hope to survive the Turks, drawing Iranian and Russian influence deeper into the region. And President Trump has demonstrated before the world that the United States cannot be considered a trustworthy partner, whether that be on an arms agreement, a trade deal or the continuing struggle against terror and extremism.

Just months ago, the U.S. president was bragging about his special relationship with Mr. Erodogan. A “tough cookie,” Mr. Trump had said, “but I get along with him; maybe that’s a bad thing.” It is fair to say now that his chummy relationship with Mr. Erdogan is indeed a very bad thing—bad for the Kurds, bad for the campaign against ISIS and bad for whatever still remains of the nation’s international credibility.

This article appears in October 28 2019.