Some things never change. Battle of Grunwald, 1410
http://en.wikipedia.org/wiki/Battle_of_Grunwald
Actually, some things do change:
http://en.wikipedia.org/wiki/Battle_of_Tannenberg
(by the way, the Lithuanians, and Lipka Tatars, neither of which were Slavs, had a lot to do with the Polish victory over the Teutonic Knights in 1410)