Four motives contend for the title of maindeterminant of international behavior: power,security, economic gains, and ideals. These four canbest be tested by comparing five American militarycommitments in Europe: World War I, World War II, theestablishment of NATO at the beginning of the ColdWar, the perpetuation of NATO after the end of theCold War, and the campaigns in the Bosnia & Kosovo. The research finds that power was the predominantfactor. As American relative power grew, the UnitedStates sought to increase control over Europeanaffairs by expanding its political interests in thecontinent. This, however, did not happen in a uniformmanner. States do not expand when their relativenational power increases, but rather when decisionmakers perceive a relative increase in state power.The theory most useful in explaining this isneoclassical realism. This theoretical perspective,extensive archival research on the beginning of theCold War, and scores of interviews with high rankingofficials in the 1990's reveal an American Empireframe of mind.