James Mattis, the secretary of defense, asserted that the Defense Department does not “do stunts.” But this was a blatant political stunt. The president crossed a line — the military is supposed to stay out of domestic politics. As many senior military retirees have argued, the forces are not and should not be a political instrument. They are not toy soldiers to be moved around by political leaders but a neutral institution, politically speaking.

Oh, some might say, presidents use troops politically all the time. And so they do, generally in the context of foreign policy decisions that have political implications. Think Lyndon Johnson sending more troops to Vietnam, fearing he would be attacked for “cutting and running” from that conflict. Or George W. Bush crowing about “mission accomplished” when Saddam Hussein was toppled. Those are not the same thing as using troops at home for electoral advantage.

Electoral gain, not security, is this president’s goal. Two of us served in the military for many years; while all troops must obey the legal and ethical orders of civilian leaders, they need to have faith that those civilian leaders are using them for legitimate national security purposes. But the border deployment put the military right in the middle of the midterm elections, creating a nonexistent crisis to stimulate votes for one party.