At first effectively neutral during World War II, the United States began supplying materiel to the Allies in March 1941 through the Lend-Lease program. On December 7, 1941, the Empire of Japan launched a surprise attack on Pearl Harbor, prompting the United States to join the Allies against the Axis powers, and in the following year, to intern about 120,000 U.S. residents (including American citizens) of Japanese descent. Although Japan attacked the United States first, the U.S. nonetheless pursued a “Europe first” defense policy. The United States thus left its vast Asian colony, the Philippines, isolated and fighting a losing struggle against Japanese invasion and occupation. During the war, the United States was one of the “Four Powers“ who met to plan the postwar world, along with Britain, the Soviet Union, and China. Although the nation lost around 400,000 military personnel, it emerged relatively undamaged from the war with even greater economic and military influence.
After World War II, the United States and the Soviet Union competed for power, influence, and prestige during what became known as the Cold War, driven by an ideological divide between capitalism and communism. They dominated the military affairs of Europe, with the U.S. and its NATO allies on one side and the Soviet Union and its Warsaw Pact allies on the other. The U.S. developed a policy of containment towards the expansion of communist influence. While the U.S. and Soviet Union engaged in proxy wars and developed powerful nuclear arsenals, the two countries avoided direct military conflict.