Final answer:
Under the Selective Service Act, nine hundred thousand men aged twenty to thirty-six were required to register for the draft, which enabled them to be potentially called up for compulsory military service during World War I and in subsequent conflicts such as World War II and the Vietnam War. Today, the Selective Service System requires all male citizens between eighteen and twenty-five to register.
Step-by-step explanation:
Under the terms of the Selective Service Act, nine hundred thousand men aged twenty to thirty-six had to register for the draft during World War I. The Act was initially passed in 1917, requiring all men aged twenty-one through thirty to register. It was expanded in 1918 to include men between eighteen and forty-five. These men were subject to being called up for compulsory military service to create a fighting force during times of war. Local draft boards exerted control over the process, which sometimes led to inconsistent enforcement and arbitrary interpretations of service qualifications and deferments.
During World War II, the Selective Service Act of 1940 required all young men between the ages of twenty-one and thirty-five to register for the draft. Men who were physically and mentally fit, single, and not employed in a critical occupation, were classified into Category I and could expect to be drafted. The Selective Service System was also the mechanism through which men were drafted for the Vietnam War, calling men between the ages of eighteen and twenty-six as needed.
Today, under the Selective Service System, all male citizens between the ages of eighteen and twenty-five are legally required to register. This system serves as a contingency in case a draft is needed, ensuring the military could be quickly supplemented with civilian forces in the event of a war or crisis.