What a fiasco. Foreign-policy blunders often take months or years to reveal their damaging consequences, but the harm from President Trump’s abrupt withdrawal of U.S. forces from northern Syria is playing out almost in real time.

Critics said Turkish President Recep Tayyip Erdogan would invade northern Syria despite Mr. Trump’s public warnings, and the Turkish strongman did. Critics said our Kurdish allies would strike a deal with Syria’s Bashar Assad to defend themselves, and the Kurds have. Critics said Islamic State prisoners...