First it was the Europeans who sought an escape from the tragic realities of power that had bloodied their 20th century. At the end of the Cold War, they began to disarm themselves in the hopeful belief that arms and traditional measures of power no longer mattered. A new international system of laws and institutions would replace the old system of power; the world would model itself on the European Union—and if not, the U.S. would still be there to provide security the old-fashioned way.

But now, in the wake of the wars in...