In 1940, as the US ramped up military preparedness, the War Department started drafting young men into the Army--the largest of the military services. The Navy and Marine Corps relied on volunteers. Michael volunteered for the Marine Corps. His deferment from the Army draft was irrelevant to the Marines because he volunteered.

Vito probably bribed, or had Tom bribe, a doctor to certify to Michael's Draft Board that Michael had some physical or mental condition that would render him unfit for military service. He might not have told Michael about it simply because he assumed that Michael wouldn't, or shouldn't, want to be drafted That was before Pearl Harbor.