https://en.wikipedia.org/wiki/Fourth_Crusade

The crusaders inflicted a savage sacking on Constantinople for three days, during which many ancient Greco-Roman and medieval Byzantine works of art were either stolen or destroyed. The magnificent Library of Constantinople was destroyed. Many of the civilians of the city were slaughtered, raped and looted. Despite their oaths and the threat of excommunication, the crusaders ruthlessly and systematically violated the city's churches and monasteries, destroying, defiling, or stealing all they could lay hands on; nothing was spared. It was said that the total amount looted from Constantinople was about 900,000 silver marks. The Venetians received 150,000 silver marks that was their due, while the crusaders received 50,000 silver marks. A further 100,000 silver marks were divided evenly up between the crusaders and Venetians. The remaining 500,000 silver marks were secretly kept back by many crusader knights.

This so weakened the Byzantines that they never became a real power again and limped on for a few generations before the Muslims finished them off in 1453 putting a final end to the Roman Empire.