Final answer:
The Selective Service Act empowered the government to draft men into the military, initiating in 1917 with subsequent expansions and modifications, and it presently mandates that all male citizens register with the Selective Service System at eighteen.
Step-by-step explanation:
The Selective Service Act authorized the U.S. government to draft men into military service. Initially passed in 1917 and applicable to men aged twenty-one through thirty, it was later expanded in 1918 to include men aged eighteen to forty-five. During World War II, the Selective Service Act of 1940 required young men to register for the draft, categorizing them based on their fitness, marital status, and the criticality of their occupation.
The act has evolved over time; today it requires that all male citizens register with the Selective Service System at the age of eighteen. However, there is no mandatory military service unless there is a military draft. The Selective Service System is critical in creating an Army in times of need and ensures that the United States can quickly mobilize a fighting force should it become necessary.