You technically have to serve out any time left on a contract (enlistment term), but there are exceptions. Extreme hardship, extraordinary circumstances or in the best interest of the service.

I saw a number of people, enlisted & commissioned who 'resigned' from the corps. Interestingly enough, it was for being gay. This was in the 80s. They were actually gay, but were confessing to it just to get out early. They were given 'other than honorable' discharges.