The core book has Range and Cover, which handles skirmishes of teams between one and about a dozen people.
The recently-released Anthology has a chunky chapter on war, from fighting battles to raising troops, to maintaining supply lines. It’s cutrently out of stock, but they will likely be doing a second prining soon. Meanwhile, this post has an earlier draft of said rules you can check out.
There is also, of course, the core mechanics and resolutions that can be brought to bear – Test Logistics to establish supply lines, test Administration to set your men into effective regiments, test Strategy vs your opponent to win a war against them. Maybe link all those tests together. Burning Wheel facilitates multiple levels of play: You can resolve a year’s worth of war fighting with a single Vs Test at the table, or crack open the Anthology and get stuck in like it’s the fucking Creative Assembly.