When I ran my first Birthright campaign, I didn't really like the card battle which I found too simplistic, and I used wargame like rules (I don't really remember which ruleset I used, but it was something close to BattleLore).

However, it didn't really improve battles, as they felt like a completely separate game (which was also a problem with the battle card system, but at least, it was quite simple, and fast). What felt the "wrongest" to me was that battles should be the perfect occasion to "bridge" adventuring and realm play, but a separate battle system doesn't work too well for this.

So I am now looking for a battle system that let the player influence the battles through their direct actions at the individual level, while still making the army meaningful, both for my next campaign of Birthright, and for Zodiac Legion, the computer strategic RPG I am working on (as this one has the same prerequisite when it comes to representing mass battles).

ACKS: Domains of War (Campaigns) seems to be close to what I want. If you went the same road, which abstracted battle system(from another ruleset, or homebrew) have you used with Birthright?