President Trump’s critics see his decision to withdraw U.S. forces from northern Syria as the product of a dangerous impulsiveness that ignores strategic realities. They argue that it betrays the People’s Protection Units, or YPG, the Kurdish force that helped the U.S. defeat Islamic State, while rewarding a dangerous autocrat, Turkey’s President Recep Tayyip Erdogan. But it is Mr. Trump’s critics who disregard reality.

Most members of America’s foreign-policy establishment see Turkey as an ungrateful ally, perhaps even a...