Being drafted into the military means being ordered to join the armed forces, usually during a time of war or national emergency. In the United States, military conscription, commonly known as the draft, has been employed by the U.S. federal government in six conflicts: the American Revolutionary War, the American Civil War, World War I, World War II, the Korean War, and the Vietnam War. The Selective Service System, otherwise known as the military draft or conscription, requires almost all male U.S. citizens and immigrants, ages 18 through 25, to register with the government. However, conscription remains in place on a contingency basis, and the United States currently has an all-volunteer military. If the draft were to be reinstated, the Selective Service System would activate and order all personnel to report for duty, and individuals would be chosen for conscription through a process of classification based on their circumstances and beliefs.