Explanation: In the medieval period, a series of wars known as the "Crusades" were undertaken by Catholic Christians from Western Europe, nominally in support of the Orthodox Christians of the Byzantine Empire, roughly modern day Greece and Turkiye. The goal of these expeditions was to reclaim the "Holy Land", a subsection of the Levant important to Christianity, from Muslims.
During the utterly botched Fourth Crusade, which ran into numerous other mishaps and misadventures along the way, a significant body of the crusaders not only utterly failed to engage their nominal Muslim enemies, but they also warred on, besieged, overthrew, and conquered the Byzantines. This despite the Pope explicitly forbidding them from warring on any Christian polities.
But hey! Loot, land, and power! What's not to like?