Obama hoped to transform the world. It transformed him.

At first glance, the twists and turns of Mr. Obama’s Syria policy made the president seem indecisive, if not incoherent: calling for Mr. Assad to step down without taking direct action against him, even after the regime’s use of chemical weapons in defiance of Mr. Obama’s “red line”; attacking the jihadists of the Islamic State while allies like Turkey and Qatar supported other extremist groups; opposing Russian designs and then coordinating airstrikes with Moscow. But the aim of keeping American troops out of Syria was consistent. At his final news conference as president, Mr. Obama expressed anguish over the fall of Aleppo, but insisted that his Syria policy had been guided by his sense of “what’s the right thing to do for America.”

It may well have been; American lives were spared. But noninterference created a vacuum that autocrats like President Vladimir V. Putin of Russia and Recep Tayyip Erdogan of Turkey were happy to fill. What’s more, Mr. Obama’s understanding of American interests in Syria was more restrictively drawn than one might have expected from a man so worldly, someone who had always stressed the interdependence of the global community and the moral burdens of “what it means to share this world in the 21st century.” Who governs Syria may not be a core American interest, but the country’s apocalyptic splintering is another matter. The effect of Mr. Obama’s caution, as much as Moscow’s belligerent resolve, was to help prolong the war.

The consequences of Syria’s disintegration have spread far beyond its borders. Not only has the crisis placed dangerous strains on neighboring states, but it has emboldened the far right in Europe, which has played on fears about Islam and terrorism in its campaign against immigration and the European Union. Nor has the United States been unscathed by what Mr. Obama recently called the “tug of tribalism”: Donald J. Trump owes his election to it. Mr. Trump is an open admirer of tribal politicians like Mr. Putin, Mr. Erdogan and Prime Minister Benjamin Netanyahu of Israel, not least because they remind him of himself with their love of the mob, contempt for liberal elites and penchant for conspiracy theory.