In a world at war, and with wars rarely being a real solution, the nuclear arms stalemate has been a realistic compromise between Catholicism’s pacificism and its just war theory.

An iconic photograph from the waning days of the Vietnam war now seems likely to be recreated in Kabul. In it, an Air America helicopter lands on the roof of a U.S. Central Intelligence Agency building across town from the U.S. Embassy. A CIA officer guides a long line of Vietnamese up a ladder and into the chopper, their last hope of escape from...