By the time the Normans and William the Conqueror invaded England in 1066, they brought with them not only a mounted cavalry but also an emerging code of honor, one that preached restraint even to those one had bested on the battlefield — a sort of medieval Geneva Conventions.
As far as I know, the verb best actually means to outwit someone. What you really do on the battlefield is you fight and kill your enemy. It has nothing to do with outwitting someone. So, I'm not sure what the author is really saying there.