Constitutional FAQ Answer #49
Q49. "It would seem that Amendment 13, Section 1 would outlaw the concept of the conscription of citizens into the military forces ("Neither slavery nor involuntary servitude....") How has our government legally justified that practice (I'm sure I'm not the first person to see this paradox.) It would also seem that, by extension, the mandatory registration for the draft that currently is practiced would also be unconstitutional."
A. Though you might be able to argue that compulsive military service is a form of involuntary servitude, I think that if the issue came up in court, the judge(s) would note that the term "involuntary servitude" has a very specific connotation, and military service is not part of it. The Congress has the power to regulate the armed forces, and the Selective Service registration doubtless would fall under that provision.