The Battle of Manzikert (Mantzikert) in ancient Armenia in August 1071 CE was one of the greatest defeats suffered by the Byzantine Empire. The victorious Seljuk army captured the Byzantine emperor Romanos IV Diogenes, and, with the empire in disarray as generals squabbled for the throne, nothing could stop them sweeping across Asia Minor. Manzikert was not a terrible defeat in terms of casualties or immediate territorial loss, but as a psychological blow to Byzantine military prowess and the sacred person of the emperor, it would resound for centuries and be held up as the watershed after which the Byzantine Empire fell into a long, slow, and permanent decline.
This is a companion discussion topic for the original content at https://www.worldhistory.org/article/1189/battle-of-manzikert/.