Muhammad Ali's refusal to sign his Vietnam-era military draft card upended the boxing champ's life and added a powerful voice to the anti-war movement.
A recent think tank report looks at the unprecedented social and cultural challenges the Defense Department would have to navigate in the event of a draft.
There's a lack of hard information on why propensity to serve is falling, and whether there's an ideological shift in attitude toward military service.
In 1971, Gravel led a one-man filibuster to protest the Vietnam-era draft and he read into the Congressional Record 4,100 pages of the leaked Pentagon Papers.
