It would be save game bloat hell to record every battle like that, but some kind of mechanic to record significant, defining battles in major wars could be potentially cool if done right.
I had one battle in a campaign a long time ago, where I was playing Germanic and took most of a weakened, vulnerable France in an opportunistic Great Holy War, and was defending the crusade that came a few years later. It seemed certain France was to be taken back by the Christians, but then I had a battle where the Pope and leader of the Knights Templar were both enemy commanders, and I was lucky enough to capture the two of them. I mutilated the Pope and happened to remove one of his eyes--making him look like the One-Eyed God and giving him a bonus to Pagan Opinion, then forced him to end the crusade, and carved the Templar who dared suffer the wrath of the Fylkir into a blood eagle.
That's the sort of history defining moment in a war that would be fantastic to be remembered ingame somehow.
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