President Trump’s decision to withdraw 1,000 American troops from Syria without consulting any aides, experts or allies, and without any warning to America’s Kurdish comrades in arms, whom he placed in mortal danger, has provided chilling evidence of the danger posed by his chronic inability to appreciate a president’s responsibilities.

Mr. Trump, as he always does, claimed a huge victory — “an amazing outcome” that saved “millions and millions of lives.” That scores of Kurdish lives have already been lost, that thousands of people have fled their homes, that a swarm of Islamic State followers escaped from internment camps, that the Kurds themselves turned for help to the mass murderer Bashar al-Assad, that America’s dwindling credibility in the world was further undermined, meant nothing to the president. “It’s not our border,” he said on Wednesday.

Mr. Trump’s apologists, too, have been quick to marshal a defense — the Middle East is full of horrible dictatorships, conflicts and crimes against humanity, and presidents before had longed to pull America out of what Mr. Trump has called the region’s “endless, senseless wars.” In northern Syria, the Americans were trapped between two allies, the Kurds who fought with them on the ground and the Turks, whose country is a NATO ally and repository of American tactical nuclear weapons. Something eventually had to give. There was a serious case to be made for pulling out.

But not like this.

The acute shame of the moment was captured in two reports this week. The first was a video of a Russian-speaking reporter wandering through a hurriedly abandoned American base in northern Syria, rummaging among the Coca-Cola cans and footballs. The second arrived with news that two United States Air Force F-15 jets had destroyed an American munitions bunker in Syria to prevent munitions and other equipment from falling into the hands of other armed groups.