Now, after another phone call with Mr. Erdogan, Mr. Trump has repeated his blunder. Again ignoring the advice of the Pentagon and disregarding allies and Congress, he ordered U.S. forces to withdraw from the area along the border between Syria and Turkey, thereby facilitating a Turkish invasion. The Turkish objective is to evict the Syrian Democratic Forces (SDF), a Kurdish-led group that has been an invaluable U.S. partner in fighting the Islamic State. Mr. Erdogan wants to send hundreds of thousands of Syrian refugees now in Turkey to the 20-mile-wide strip his army intends to occupy.

The consequences are likely to be the same as Mr. Trump was warned of before. Betrayed by the United States and forced to fight a potentially bloody conflict with Turkey, the Kurdish-led forces could quickly abandon any further effort to control the Islamic State. They might well set free the tens of thousands of former militants and family members held in SDF-controlled camps. The 1,000 U.S. troops in Syria could be forced to withdraw entirely, which would be a major victory for Russia and open the way for Iran to entrench its forces along Israel’s northern border. U.S. allies around the world meanwhile will have reason to question whether they should cooperate with a government that so casually abandons military partners.

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The White House statement announcing Mr. Trump’s decision and the presidential tweets that followed it reflected a stunning ignorance of the situation on the ground. Mr. Trump claimed that the invading Turkish forces would take care of the thousands of imprisoned Islamic State militants. But the camps that hold them are outside the area Mr. Erdogan plans to seize — and, in any case, the Turkish army “has neither the intent, desire, nor capacity” to take on that job, as the former U.S. envoy to the region, Brett McGurk, tweeted.

Mr. Trump claimed to be fulfilling a mandate to stop “endless wars” in the Middle East. But unlike the large-scale U.S. missions in Iraq and Afghanistan, the Syria operation was a light-footprint, low-cost operation — and a striking success. With just a few thousand troops and air power, the United States was able to partner with the SDF to destroy the would-be Islamic caliphate and gain de facto control over a large swath of eastern Syria. That impeded Iran’s expansion in the country and gave Washington vital leverage over any eventual settlement of the Syrian civil war. In five years, 17 U.S. soldiers have been killed in action in Iraq and Syria combined.

The president claims that the Islamic State has been eliminated, but it has not. He suggested Turkey, Russia, Syria and European countries would prevent the group’s resurgence; they won’t. He also boasted that “if Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the Economy of Turkey.” Such unhinged rhetoric ought to worry even those Americans who support Mr. Trump’s decision.