Sooner or later, in many RPG campaigns there’s temptation to throw the PCs onto a battlefield. Massed troops, war machines, death, carnage, destruction…it’s a glorious excuse for a big action scene.
The question is, how do you handle a mass battle?
The answer is actually surprisingly easy.
Do it in exactly the same way as the movies do: maybe throw in the occasional shot of the overall battle, but for the most part focus on the main characters and what they’re doing in the battle.
If the game is one where the PCs are competent, then that’s going to be leading charges, flanking the enemy or using the battle as cover to get to someone/something.
If the game is one where the PCs are legendary heroes approaching demigod status, then have them cutting down dozens of enemies.
If the PCs split up (more likely if they’re at the demigod end of things) then what I’d probably do is give a character a couple of rounds of action, then cut to the next character for a couple of rounds.
Look at how the Avengers movie did things; you saw what one or two characters did, then one of them would meet up with or pass another character and then you’re following them for a bit. You don’t get to see what any of the Avengers are doing for the entire fight, but it’s still clear that they’ll be kicking ass even when they’re off screen. Do that. Maybe you won’t get the nice links between each bit as the PCs are way across the battlefield from each other. That’s okay, we can always just cut to other characters.
Don’t let things get bogged down in what’s happening in the big picture: a battle is generally a confusing thing to be in the middle of, so don’t give them too much time to think and keep things moving.
Unless they have a chance to step back from the fighting to a vantage point (or have some sort of tech/magic to give the info) the PCs probably won’t know how things are going until it’s a forgone conclusion anyway.
The players are there to play their characters
If you have the players watching the troops duke it out then that’s no better than having them watch two NPCs fight: it’s dull, and it’s passive. Maybe under certain circumstances it might work as a narrative tool, but in general it’s the tabletop equivalent of a video game cut-scene (and I think we’ve all skipped a few of those in our time). Players want to be actively involved.
If the players are given control of troops, that might work, but it’ll depend entirely on your group’s buy-in for that. If they aren’t particularly interested in the troops, or aren’t too bothered about wargaming then it’s going to suck.
I made this mistake once and that session totally bombed (despite a couple of the players actually being wargamers and me using light fast-play rules).
The players should be allowed to be their characters, and just their characters. If that involves leading a unit of troop or giving orders, that’s cool, but it probably should just be them doing exactly that: having their character lead and give orders. Keep what the troops do and how the battle is going as pieces of narrative. The focus should be on the PCs.