Is it possible to serve in multiple branches of the military ?

I started out in the Corps and Aviation was getting over strength and was told to become a grunt 0311 or get out; I crossed over into the Army Aviation and continued for another 17 years

Start with the USAF they are the most picky,The Army is gruelling and hard, I suspect the Marines are close,HOWEVER the NAVY has an EXCELLENT education system A LOT easier basic than those two as well. ...however the AIR FORCE won't take anyone else in their ranks as I recall.

I did it 15 yrs in the Marine Corps and 15 yrs in the Army.

What everyone has said is all good but some branches will not take you if you were a marine

ummmm....yes. You can serve in any branch for which you qualify. I know several folks who have jumped between the Air Force, Marines, and Navy.

I served in the USMC 93-03 NAVY 04-08 ARMY 08-14. Retired as SSG.

I don't see much advantage to serving in multiple branches. As you progress in experience and rank there's not much to gain and much to actually lose by jumping branches. Find what you love and make it your own. However IF you do decide to jump branches, do it as early as possible to minimize the impact on your career progression.

I am currently serving in my 3rd branch, Navy, Army, now AF... commissioned in the Army in 1990! I have a friend that stayed Army that is soon to pin on General, I am soon to pin on LtCol. You lose some by switching branches but you also gain a lot of appreciation for service in the military as a whole. Balance your life with the mission of each service but consider serving as a privilege regardless of the branch you serve... cultures are different in each branch. I am often asked which branch I like the best, honestly each one was excellent for that time period of my life. But, consistancy might have served me better...

Two ways to "serve" in other branches;

1-after completing an enlistment in one branch, enlist in a different branch. I served in the Air Force and then the Army and know many vets who've served in more than one branch.

2-Take an assignment to a joint command. Joint commands are staffed by personnel from two or more branches and work together for a common mission.

I'm not aware of any method of cross training between branches because each branch has it's own particular organization and mission.

Yes it is one at a time. I started out in the Navy Seabees in Gulfport Mississippi and ended my career in the Army NationalGuard deployed to Iraq from Hattiesburg Mississippi.