THE UNITED States has been the dominant world power since 1945, and U.S. leaders have long sought to preserve that privileged position. They understood, as did most Americans, that primacy brought important benefits. It made other states less likely to threaten America or its vital interests directly. By dampening great-power competition and giving Washington the capacity to shape regional balances of power, primacy contributed to a more tranquil international environment. That tranquility fostered global prosperity; investors and traders operate with greater confidence when there is less danger of war. Primacy also gave the United States the ability to work for positive ends: promoting human rights and slowing the spread of weapons of mass destruction. It may be lonely at the top, but Americans have found the view compelling.