The greatest tragedy of the second millennium (1000 AD -2000 AD) was the fall of the Byzantine empire

 The greatest tragedy of the second millennium (1000 AD -2000 AD) was the fall of the Byzantine empire (the richest and most powerful empire ever know). Just before the fall the  Byzantine rulers and their clergy had agreed (at the council of Ravenna) to Reconcile with The Holy Church of Rome. The  Byzantines refused to honor their agreement and soon after their great empire was no more.