Can you serve in multiple branches of the US military?

Answer 1

No. You can be in one branch of the military, and be attached to another branch (for example, Air Force Tac-P personnel are often assigned to Army or Marine Corps units), but you can't be in two branches simultaneously.

Answer 2

You can only work in one branch of the military at a time, because it is a full time job. You could, in theory, join one branch and then quit and join another, until you have been in all four, although this would give you a very peculiar job history and might arouse suspicion about what you are trying to do. Only the President, who is the Commander In Chief, simultaneous plays a role in all branches of the military, being the head of all of them. So your best bet is to become President.

Answer 3

Not at once. You can go into a different branch once your current contract expires, or there are ways to transfer from 1 branch to the other.