There's a sci-fi series where the only difference between a Marine and a Soldier is where they're stationed. Same training, same everything, it's just that when they're planet side they're "Army", and when they're on a space ship they're "Marines".
Something similar here would probably work, assuming you allow the Marines to either basically take over the Army or to carve out a good sized fiefdom so they don't get shuffled off into a corner or disbanded.
The mission of the Marines is supposed to be to secure the beachead long enough for the Army to bring its train of troops and supplies to begin the sustained battle. They are not supposed to be involved in long-term prosecution of dipliomatic goals by other means - although they have been tasked with doing so since Nov. 11, 1775. "Never have so many done so much with so little" - hah!
The problem with the Marines is that they have grown too large and have been given too many toys. IMHO there ought to be three divisions (Atlantic, Pacific, and Indian Ocean) that fit entirely on three fleets of ships that may or may not be attached to the USN's other fleets. Cruise and stop off at liberty ports until the balloon goes up. (Not only does this plan have strategic sense, it offers a cure for the world's falling economy - the Marine on leave in a foreign port,
)
stay safe.
