Saracens and Franks!

Explanation: During the Crusades, the two quarreling sides - the predominantly Western European Catholics, and the predominantly Muslim inhabitants of the local polities - referred to each other as “Franks” and “Saracens”.

However, “Franks” originally referred to a specific German tribe which settled in… well, what is now France. Yet the Crusaders drew from all across Europe! And likewise, “Saracen” comes from an old Latin term for Arab tribes, while the actual anti-Crusader coalition was much, much broader than just Arabs.

Truth is truly the first casualty in war!