Army Diplomacy demonstrates how, in the immediate aftermath of World War II, the United States Army became the principal agent of American foreign policy. The army designed, implemented, and administered the occupations of the defeated Axis powers Germany and Japan, as well as many other nations. Generals such as Lucius Clay in Germany, Mark Clark in Austria, and John Hodge in Korea presided over these territories as proconsuls, and at the beginning of the Cold War more than 300 million people lived under some form of US military authority. This massive occupation effort had roots in a century of army practice, especially influenced by the army’s Rhineland occupation. The army policies in the occupied nations also represented the culmination of more than a century of military doctrine. Army Diplomacy relies upon institutional history, military sociology, and international relations theory to show how the army’s institutional history and doctrine led to development of post–World War II occupation governance that reflected the particular imperatives of the US Army, especially the army’s requirement that all matters of governance be subordinate to requirements of military necessity. Army Diplomacy further shows the army’s bureaucratic skill in winning the intergovernmental debate over postwar governance against other US government rivals. Finally, Army Diplomacy reveals how the implementation of military government in postwar Germany, Austria, and Korea not only informed but also profoundly influenced early US Cold War policy.