Manzikert

On this day in 1071, the Seljuk Turks defeated the Byzantine Empire at the Battle of Manzikert, largely due to the treachery of the Byzantine second in command. The battle led directly to the decline and eventual downfall of of the Byzantine Empire, and indirectly to the Crusades; consequently, I consider it one of the most important battles in history.

This is also the date of the Battle of Crécy.