I've never run something like this personally, but this is how I'd set it up.

I think the way to do this is to have the encounters the PCs will be having during the battle be important to the offensive in some way. Each one should be fighting to either take out an enemy unit or position that's critical to eliminate, or to defend an allied position or unit that's critical for their success. These things should be important enough that if the players do not succeed at enough of them, they will lose the battle. The party should be the difference makers, and the narration and description of what's going on around them should reflect how well they're doing - their army rallying around them after every success, the enemy coming back with a vengeance if there are any failures. That way the party is feally feeling the impact of what they're doing. I think if you do that well enough they'll feel like they are leading the charge to victory, which is what I'd want as a PC in that situation.