Siege of Constantinople (1203)
The siege of Constantinople in 1203 was a crucial episode of the Fourth Crusade, marking the beginning of a series of events that would ultimately lead to the fall of the Byzantine capital. The crusaders, diverted from their original mission to reclaim Jerusalem, found themselves in Constantinople, in support of the deposed emperor Isaac II Angelos and his son Alexios IV Angelos.
Source: Wikipedia — Siege of Constantinople (1203) (CC BY-SA 4.0)