There is a "War Machine" rules system in the BECMI Companion ruleset for covering mass battles.
I've used this a little bit, and personally I think it's a fun and interesting subsystem.
However, it depends on what you're looking for...
The BECMI subsystem is meant to determine the outcome of a mass battle, but it does not include the PC's actions. If used as written, it's mostly about the DM calculating stats for the rival armies and rolling dice every turn to see how many troops are killed or flee, while the players watch.
However, there is a few tweaks you can apply to make the subsystem a lot of fun:
1. Let the PC have a role in the preparation phase, e.g. gathering and rallying the troops, arranging the equipment, making battle plans etc... Mostly this falls in the "interaction pillar" of the game, but these are things that have an effect on the army stats.
2. If the PC are mid-high level (or otherwise find another explanation), let each of them be appointed as commander of one army, so that the player will directly choose the tactic of that one army at each turn. The DM runs the rest of the armies (i.e. the remaining armies on the players' side and all armies of the enemy).
3. Don't let the PC die in battle, since the players aren't controlling them directly, just assume that when you roll for number of casualties, the PCs are not among them. (You can be less forgiving if you want, e.g. you can add a % chance for PC death each turn, or perhaps only if her whole army is destroyed)
I also think that you can improve this further by including some "cut-scene" where one or more PCs are directly involved in a fight using regular combat rules, but IMO this may also slow things down too much and you kind of lose the novelty of a mass battle.
BTW, if you don't have BECMI books, you can look up these rules from the free Dark Dungeons clone (it only has some minor changes to the original).
