The Byzantine Empire
The Byzantine Empire was the Eastern Half of the Roman Empire that did not fall to the barbarians as Rome did in 476 AD. In the 11th Century, the Byzantine Empire began to be threatened by the Seljuk Turks and fought a series of wars with them.
In 1071, the Byzantine Empire fought The Battle of Manzikert against the Seljuk Turks. This battle proved crucial, with the Byzantine Empire being quickly reduced from this size in 1060 AD, to this size a mere 10 years later.
Following this string of defeats, the Byzantine Emperor
Alexius requested assistance from the Western Christians.
This request culminated in the First Crusade, although the
assistance that the Western Christians provided was not
quite what the Emperor had expected.
The First Crusade was significantly beneficial for the
Byzantine Empire because it allowed the Empire to take
back much of the territory that it had lost to the Seljuk
Turks.