Follow the Money

Religion provided the justification, but economics drove participation:

• Younger sons of noble families, who inherited nothing under primogeniture laws, saw crusading as a path to land and wealth
• Italian merchant cities — Venice, Genoa, Pisa — financed crusades in exchange for exclusive trading rights in conquered ports
• The Fourth Crusade (1204) never even reached the Holy Land. Venice redirected it to sack Constantinople, a Christian city, to eliminate a trade rival

The sack of Constantinople was so thorough that the Byzantine Empire never recovered. Crusaders stole relics, melted gold from churches, and divided the empire among themselves.