Kurds and Syrians took nearly all of the ground casualties in the previous fight against the caliphate. Why would they do so again after Mr. Trump abandoned them against the Turks? And especially after Mr. Trump said this week that the Kurds might have helped against ISIS but they were well paid and hadn’t helped us at “Normandy”—as in D-Day…

As Commander in Chief, Mr. Trump has been mostly tactical and rarely strategic. He shifts positions from week to week, even day to day, for the sake of a summit or short-term appearances. Allies are informed about his reversals after the fact and left to wonder if they can still rely on the United States of America.

As Mr. Trump runs for re-election, this habit of impulsive judgment will be front-and-center. As an incumbent he should be the safer presidential choice. But Mr. Trump’s judgment can be so reckless that many voters who took a risk on him the first time will ask if he’s worth a second gamble when he would no longer be disciplined by having to face the voters again. Impeachment won’t defeat Donald Trump in 2020, but Donald Trump might.