How about AI "fleets" of vehicles that drive around the frontlines engaging each other in a never ending, futile slugging match?
Only untill players join in the fight does the AI frontline move. Untill then all the little AI units do is manage a 1 for 1 kill ratio against each. They act like lemmings pooring out from the supply line end point heading right for each other. Now if you add in the players and their firepower that AI stream should gain the numerical upper hand and push forward.
Hey, I like the army. Who wouldn't want to see a battle raging?
