How did the military recruit the four million americans who served during world war i

:  Only about 2.8 million were drafted under the Selective Service System 
All males aged 21 to 30 were required to register for military service for a service period of 12 months. As of mid-November 1917, all registrants were placed in one of five new classifications. Men in Class I were the first to be drafte and men in lower classifications were deferred. Dependency deferments for registrants who were fathers or husbands were especially widespread.[9] The age limit was later raised in August 1918 to a maximum age of 45. The military draft was discontinued in 1920