With the fall of the Iron Curtain, the demise of the notion of a "Pax Sovietica", and the end of the Cold War, the U.S. maintained significant contingents of armed forces in Europe and East Asia. The institutions behind the Pax Americana and the rise of the United States unipolar power have persisted into the early 21st century. The ability of the United States to act as "the world's policeman" has been constrained by its own citizens' historic aversion to foreign wars. Though there has been calls for the continuation of military leadership, as stated in "Rebuilding America's Defenses":