Rather, Obama regrets not getting busy “on the ground” to “manage” Libya’s transition to democracy. So, even after 13 years in Afghanistan and nearly a decade in Iraq, Obama wishes America had gone into Libya for more of the excitements and satisfactions of nation-building.

Two questions must be distinguished. First, is it an important American interest or duty to protect, as much as air power can, Kurds and Yazidis from the Islamic State, and to (in Obama’s words) “push back” (back to where?) this group? Even if the answer is yes, there is another question: Is it wise to support the use of force by this president? He is properly cautious about today’s awful dilemma, which is not primarily of his making. But caution can be reckless.

One of Napoleon’s aphorisms — “If you start to take Vienna, take Vienna” — means: In military matters, tentativeness is ruinous. Are F-18s going to be used for a foreign policy of right-minded gestures — remember #BringBackOurGirls? — the success of which is in making the gesturers feel virtuous? “Success,” said T.S. Eliot, “is relative: It is what we can make of the mess we have made of things.” There is much material — rubble, actually — to work with as we seek success.