During the Vietnam war, the draft was active. Were young men drafted into the Marines, or did the Marines remain volunteer only? I ask this because the Marines are a lot harder for recruits to handle during boot camp than the Army, for example. It seems like it would be "less fair" for a young man to be forced to go through Marine boot camp, when others were only forced to go through the Army bootcamp.
Was the draft present for the Marine corp? If so, how did it work?
Yes draftees could be sent to any of the branches, but the vast majority ended up in the US Army.
And despite modern revisionist history by the Marine Corps, and popular culture, all branches and formations which deployed to SE Asia suffered from collapses in discipline, morale, and elan as the war dragged on. The warrior mythos of both the Army and Marine Corps took serious hits in Vietnam, despite some notable victories for both branches.
And despite the popular image that the Corps has implanted of the Hell that is Paris island or Recruit Depot San Diego, both during the Cold War and after the basic physical standards and minimums remained relatively similar across branches.
For instance the modern USMC PFA in use for decades consists of Pull Ups, Sit Ups, and a 3 mile run, with the minimums for an 18 year old kid being 3, 50 in 2Min, and 28 minutes to finish.
While the modern Army PFT includes push ups instead of pull ups and a 2 mile run. The minimums being 42, 53, and 15:54.
Both of which are vastly different form what the services sued during WW2, which was a more involved but arguably harder assessment. Consisting of Pull Ups, Squat jumps, pushups, sit ups, and a shuttle run.
A neat paper I came cross several years back was this by then Capt Thompson, USMC, and his thesis from the Naval post Graduate School on the history of Physical Fitness in the Corps. http://www.dtic.mil/dtic/tr/fulltext/u2/a443310.pdf