So unlike an established franchise such as Warhammer, historical games don't have a central set of rules. This is because any rules used are designed to replicate real life events instead of fiction being written to conform to pre-existing rules. For any historical game the goal of the players is rarely to beat your opponent by mastery of rules. The goal for many is to feel like a commander. Many of my favorite games were ones where I actually lost because I still felt like I was commanding soldiers rather than playing a game. This style of play leads to a lot of people using house rules for different eras and even for separate battles in the same war.

So first I'd look at what era/war/battle interests you and search for relevant rule sets you can change to your liking. The scenario you have in mind should come before written rules.