Not even Santa Claus is safe from Donald Trump’s slander. The president, 79, took calls from children across the United States on Wednesday as part of the North American Aerospace Defense Command’s Santa tracker—during which he warned one child about a “bad Santa” infiltrating the country.

Christmas under Donald Trump brought air strikes abroad and political threats at home, as the US president used the holiday to project a vision of power rooted less in peace than in grievance, even as...