The United States fought five wars in the 1900s: World War I, World War II, the Korean War, the Vietnam War, and the Gulf War. In 1917, 2 million American soldiers went to France to help end World War I. The United States joined France, Britain, and Russia in a war against Germany and Austria-Hungary. The war ended in 1918. In 1941, the United States entered World War II. The United States joined Britain, the Soviet Union and other allies against Germany, Italy, and Japan. This war ended in 1945. The United States fought the Korean War in 1950. The United States and its allies wanted to stop North Korean communists from invading South Korea. The war ended in 1953. Korea divided into two countries, North Korea and South Korea. The Vietnam War began in 1955. The American military started to fight in 1959. The United States wanted to stop communism in Vietnam. United States' soldiers left Vietnam in 1973. Vietnam soon became a communist country. The United States fought the Gulf War in 1991. Iraq invaded its neighboring country, Kuwait. The United States and its allies fought Iraq's armies. Iraq's armies left Kuwait just a few months after the war began. The United States fought five wars in the 1900s: World War I, World War II, the Korean War, the Vietnam War, and the Gulf War.