as vagabond said, its all volunteers

They join the armed services (army, navy, air force, marines, coast guard?, national guard) of their own free will and generally serve 4 years by contract, then their free to choose whether they'd like to stay in the services or move back to civilian life.... TMK anyways there maybe other deals with the GI bill etc.
(GI Bill, is when you serve your time in the military and the gov't gives you a sum of money to pay for university etc.)