TL;DR;
I want to model a strategic battlefield fight for my players to command and participate. I'm seeking advice on how to model it mechanically so that it is exciting for the players.
Background
The final scene of Act II of my campaign is envisioned to be a big battle between the army of the Big Bad and the allied forces gathered by my players. The full set of Act II runs with the players gaining the allegiance of the different factions to support them on the fight.
Now, in terms of that final scene, I'm struggling to decide how to DM the fight. In games and movies the action is made epic by having cinematic scenes of the fight happening with cuts to our heroes doing their amazing things, be it just plowing through dozens of enemies in quick succession or fighting the big ugly overpowered monster that is otherwise cause havoc among the army. I want that feeling
The set up will involve an army of several thousand enemies agains the different forces gathered by my players. Depending on what they've done during the second act, they'll have more or less support with a minimum number of regiments belonging to several faction (i.e. the silver guard of Wizardwight).
Intended way of DMing the scenario.
What I've come up with is to have the players be military leaders. Draw the map of the battlefield and have them manage the battlefield strategically and tactically, distributing the units and, somehow, enabling a dice based mechanism to evaluate how well each unit is doing. This will transform the tabletop into something similar to Warhammer Fantasy. They can choose to join a particular place of the battlefield to help either together or separately.
I have absolutely no idea on how to model this mechanically (as far as I know D&D 5e does not cover this kind of fights).
Question:
What are good ways to mechanically model this kind of strategic army combat? Should I switch to a different system just for this scene or can I model this with D&D 5e?
Notes on level: At this point I expect my players to range between level 9-12 and for the party to be between 3 and 4 players (this is not a limitation of the campaign, more like the players I'm running the campaign with).
Best Answer
Unearthed Arcana: Mass Combat
Disclaimer: I have not used this myself, it was however prototyped by the designers of D&D.
The 2017 Unearthed Arcana article: Mass Combat provides means of emulating large scale battles using the standard (with minor modifications) combat rules of D&D. I'm going to give the key features of that article here.
Creatures are grouped together into a certain size based on CR and number
Mass Combat rounds span 1 minute each
Mass Combat fighting is based on number of creatures and CR
Character style combat, both against single creatures and against hordes are provided for. (The latter recommends mob rules from DMG)
This article was published in early 2017, but such rules was not included in Xanathar's Guide to Everything. As far as I know there exist publicly available results from the survey gathered on the material.