World War I (1914-1918): The United States entered World War I in 1917 on the side of the Allied Powers, which included Britain, France, and Russia. The United States' involvement in the war was a major factor in the Allied victory, and the war helped to establish the United States as a global superpower.
World War II (1939-1945): The United States entered World War II in 1941 after the attack on Pearl Harbor. The United States played a major role in the war, which ended with the defeat of the Axis Powers, including Germany, Japan, and Italy. The war helped to solidify the United States' position as the world's leading superpower.