There is a very interesting interview(well discussion really) with GRRM and Bernard Cornwell where they discuss their influences
Martin has already explained that a big influence was a French historical book series (which I can't be bothered to look up to find the name), so you'd need to throw that into the mix. The way the "Lannister's Mad Dogs" operate is probably taken from how the English operated in France during the Hundred Years War, for example. (People tend to gloss over that bit when talking about Crécy and Agincourt.)