There is a working assumption among the American people that a new president enters the White House free of responsibility for the errors of the past, free to set a new course in any program or policy, and therefore free—at the very least in constitutional theory, and perhaps even really and truly free—to call off a war begun by a predecessor. No one would expect something so dramatic on the first day of a new administration but it remains a fact that the president is the commander in chief of the armed forces, and the power that allowed one president...