The rules for armies in many fantasy games come from way armies waged war in real life. You had your rank-and-file soldiers who would move together in a column. There'd be archers or other fire-support units off to the sides. Cavalry would be a highly-mobile element in the battle. Artillery would do what it could to soften up enemy lines. So it seems only fitting that you could take fantasy army rules and use them to play historical miniatures games. At least, the folks at Mantic think so. They converted their Kings of War rules to work with historical forces and they've got the book for that up for order.
Besides the rules for this version of the game, there's also 30 new army lists. They range from back in antiquity to the Middle Ages. There's even themes within those lists, as well as mercenaries that could be purchased for those forces. Also, the lists include things that aren't entirely 100% historically accurate, but bring in mythological and legendary creatures and units. These rules are compatible with "original" Kings of War, so feel free to pit your 14th Century French (or whatever) versus bands of Elves.