The most surprising thing about President Trump’s decision to overrule his top advisers and withdraw U.S. forces from Syria and Afghanistan isn’t that it was improvised and disruptive. Sudden shifts are part of Mr. Trump’s method, and disconcerting senior officials is one of his favorite management tools.
The surprise is that for the first time, Mr. Trump made a foreign-policy decision that divides the coalition that brought him into the White House and risks his control of the GOP. Mr. Trump has frequently challenged and…