So, write in some deaths. How hard is that? Even in a relativly peaceful invasion some sniper is going to get a lucky shot off or some guy will step on a landmine going off to take a leak. Even if you don't lose any troop units in the invasion, every troop unit doesn't corelate to one marine does it? I figured your troop unit was the drop ship with a platoon or whatever. So the platoon survives, but a couple guys die. A couple of JAFR's come in. The Captain gets promoted to Colonel, the Lieutenant goes to Captian, everybody moves up till the music stops playing.
The unit goes on and on, but the guys in the unit change slightly from battle to battle.
Geoschmo