And warfare almost always involves a clash of ideas, which makes moral and ideological clarity a vital part of it. That is why presidents are at pains to explain why we fight, including what we are fighting against, and what we are fighting for. Moral language is inescapably, and rightly, part of this. In his 1943 State of the Union address, Franklin Roosevelt said “In this war of survival we must keep before our minds not only the evil things we fight against but the good things we are fighting for.”

Such moral clarity has pragmatic virtues: it helps sustain support in a democracy and morale in an all-volunteer army. More importantly, it explains why we think we are right to kill while the other side is wrong to do the same thing. It is our moral theory of the war, our explanation to ourselves for why it is necessary. It is our justification before history and its Author for unleashing the terrible scourge of war. That the president’s moralizing might lead to mission creep and tempt us to try to defeat the group is, in my view, less a weakness of his policy, as Boyle thinks, and more a virtue.