On one level, Obama’s appreciation of nuance and complexity equips him well to confront the two-front crisis. For instance, his decision two years ago not to arm the Syrian rebels is looking smart in retrospect; if he had, large numbers of those weapons would have fallen into the hands of ISIS. Obama is what was once called a “long head”—a leader who patiently tries to think a few moves past everyone else. This is a good thing. Thinking hard before reacting is usually the wiser course.

But at a certain point, reacting is another word for losing. Presidents must act at least as much as they react; they must seize the initiative and thrust their enemies on the defensive. Sometimes threatening war is the only way to keep the peace. Obama knows this abstractly, but it’s at odds with his interpretation of history and his assessment of the mission of his presidency, which is to end wars, not start them.

To put it in terms of compelling historical metaphor, Obama is a “Guns of August” guy. The book of that title, by Barbara Tuchman, chronicles how bluster and a series of miscalculations led Eu