This chapter focuses on theories of Hans J. Morgenthau, a German émigré specialist on foreign relations. In the years immediately after World War II, Morgenthau emerged as the highest intellectual authority on international relations in the United States. His theory, which became known as “realism,” explained why the United States had no choice but to oppose the Soviet Union and China and prevent them from expanding their power in Europe and East Asia. However, Morgenthau also opposed U.S. intervention in the Vietnam War. This dual position marked both the high point of the German–American symbiosis and the moment of its crisis.