The Crusades began after Byzantine Emperor Alexios I Komnenos appealed to the West for military aid against the Seljuk Turks.

The Byzantines expected a small force of mercenaries but were surprised by the arrival of the massive, independent armies of the First Crusade.

Relations between the Byzantines and the first crusaders were tense, marked by mistrust and cooperation of convenience.

The Byzantines required the crusader leaders to swear an oath to return any former Byzantine territory they reconquered.

The establishment of independent Crusader States in the Levant created a new source of political friction.

Relations deteriorated further during the Second and Third Crusades.

The most catastrophic event was the Fourth Crusade in 1204.

Diverted by the Venetians, the crusaders attacked and brutally sacked the Christian city of Constantinople.

They established a 'Latin Empire' on the ruins of the Byzantine state, which lasted for about 60 years.

The Fourth Crusade dealt a devastating blow to the Byzantine Empire, from which it never fully recovered.