On March 9, 2015 President Obama formally declared a new national emergency. Most Americans went about their business that day without ever suspecting that what the White House called “an unusual and extraordinary threat to our national security” had suddenly popped up. It wasn’t as though the United States was insufficiently engaged in war at the time. That very day the U.S. Central Command announced that in the period of just a few hours beginning the previous day it had conducted five new air strikes in Syria and nine in Iraq as part of Operation Inherent Resolve. At the time Saudi Arabia...