What’s truly distressing is that until Trump’s sudden turnabout, the United States had something of a virtuous cycle going in the region. Not only was the Islamic State almost extinguished, but in addition, U.S. power was creating conditions for future stability. The new Iraqi government was eager to be a partner; Iran was realizing it had overreached in Syria; Sunni Arab allies such as Jordan, Egypt, Saudi Arabia and the United Arab Emirates wanted to help contain Turkish power and create a more stable Syrian state.

Trump aborted this positive momentum. He ceded power in northern Syria to Turkey and its proxies, which have made a ruinous mess everywhere in Syria they’ve tried to control. He abandoned a Syrian Kurdish ally that had imagined that sacrificing lives at the United States’ request would count for something. It didn’t.

Trump’s Syrian legacy: He has proved even more irresolute than his predecessor, Barack Obama. How does that feel, Mr. President?