Out of Pod Experience

you assume it would be only one branch, but this is incorrect

occupying and operating in a battlespace is how the US militaries are separated, but there's more to it than simply floating in space

planets would be used as bases of operations, similar to how foreign land masses are garrisoned today.

you need a shock troop/ground force once you've landed : marines
you need to occupy the land area to maintain it: army
you need to maintain air- and space-superiority: air force
you need flotillas to project firepower: navy

navy and air force both have planes, and in the case of the JSF they might even share airframes. the major difference between the two, is the navy's air capability is a support role for carrier groups.

another concept that ties army and air force together is: planes are an extension of artillery. so in terms of delivering munitions by range, it goes: rifle, cannon, rockets, missiles, planes with bombs.

coast guard can pass out candy or something. direct traffic maybe. little known fact: Coast Guard is Department of Transportation. Army, Navy, Air Force, and Marines are Department of Defense.