The main cause of tensions between the U.S. and USSR during the Cold War was the ideological clash between capitalism and communism. Following World War II, the two superpowers emerged with opposing political and economic systems, leading to a fierce rivalry. The U.S. sought to contain the spread of communism, while the USSR aimed to promote it worldwide. This resulted in various proxy wars, the arms race, and the formation of military alliances such as NATO and the Warsaw Pact. Events such as the Cuban Missile Crisis and the Berlin Blockade intensified the conflict, making the Cold War a defining period of the 20th century.