I'm pretty sure the definition of "Eastern Europe" is "the part of Europe where they didn't use much plate armor." I think I read that on the Internet.
OgreBattle wrote:This question is also about the kind of environment where plate armor stops being viable.
I get the general impression in looking through history that the easier a soldier is to replace, the less likely they are to wear good armor. Sending some Franks on a Crusade to liberate Jerusalem was a huge undertaking, and replacing any losses was next to impossible, so even a half-wit would have tried to suit them up like walking tanks. Someone defending against that kind of invasion would have to ship reinforcements a couple of miles, and probably outfit them with the equipment harvested from corpses, so treating each soldier as a precious resource seems pointless. Also, landed soldiers are important to the fabric of society, so knights and lords and such would be supported at least to an extent that helped ensure their survival. Samurai were probably in a similar position, socially. If your society is based on killing as many people as possible to appease the blood god then your macuahuitl-wielding soldiers are going into battle wearing wool and they can just handle getting killed as fast as possible on their own. I know steel plate armor wasn't an option for them, but their armor wasn't even good considering their technology level.
Exceptions of course exist. Mongols didn't invade with armor appreciably better than their victims, but while this might have something to do with philosophy it didn't really matter because they were simply too fast and deadly to have to deal with defending against very much. "If you never get hit you don't need armor" sort of thing. As a whole army of rogues with the Mounted Combat and Dodge feats, they probably had as good an AC as the knights they conquered.
All guesses though, I'm no scholar.