Well, I'm not sure it's as easy and just "signing up." About 10 years ago when I graduating from college, I was thinking about going the Army OCS (Officer Candidate School). However, I was diagnosed with Hodgkin's Disease (cancer of the lymph nodes). A few surgeries and 5 months of chemo later, I was told that I have a lifetime ban from military service. I would defend my country if needed. However, if you're in the military you don't get to choose, which I've noticed some people putting qualifiers on their service.