Some programs will allow past credits and some only if they were earned within 5 years of reentering, but since you were in the Marine Corps, they may give you a waiver since they would not consider it as dropping out and then reentering. The only way to know for sure is to contact the university, explain your situation, and ask for their policy. Different schools and programs will have different policies. Why not check to see if your university offers on-line or correspondence courses so that you can continue to earn credits while still in the service? Contact your university advisors and get the specifics from them.

hard sciences such as physics and chemistry, et.c. don’t “last” as long since new breakthroughs come pretty quickly in these fields. the best way to find out is to check with the college where you’re thinking of applying. they can tell you if your credits are still good.