Washington seems to be hitting numerous walls these days when it comes to its foreign policy. How do you preserve European institutions when each member will logically put its interests ahead of the bloc in fighting migrant waves? How do you stem a migrant crisis when the dissolution of Sykes-Picot boundaries creates massive power vacuums for militants to fill? How do you enforce a lasting cease-fire in Syria when Russia is still holding out for concessions from the West on sanctions and Ukraine? How do you get Kiev to agree to recognize elections in eastern Ukraine when the government can barely stand on its own feet?...