My father-in-law was a Quaker (or from one of the related pacifist groups), who broke with his family and enlisted in the Navy, after Pearl Harbor.
I've also read -- and this was a while ago -- about a Quaker who enlisted as an unarmed medic. He received the Medal of Honor for rescuing injured Marines from the front lines of one of the island battles against the Japanese; but he never carried a weapon.
I think I read that also. Maybe on the MoH website.