So a lot of people volunteered for the Fourth Crusade, more than 35,000, and the generals didn't want to march them all the way across Anatolia because they knew from experience that it was A. dangerous and B. hot. So they decided to go by boat, which necessitated the building of the largest naval fleet Europe had seen since the Roman Empire. The Venetians built 500 ships, but then only 11,000 crusaders actually made it down to Venice because like, oh, I meant to go, but I had a thing come up, etc. There wasn't enough money to pay for those boats, so the Venetians made the crusaders a deal. Help us capture the rebellious city of Zara and we'll ferry you to Anatolia. This was a smidge problematic, crusading-wise, because Zara was a Christian city, but the crusaders agreed to help, resulting in the Pope excommunicating both them and the Venetians. Then, after the crusaders failed to take Zara and were still broke, a would-be Byzantine emperor named Alexius III promised the crusaders that he would pay them if they helped him out, so the excommunicated Catholic crusaders fought on behalf of the Orthodox Alexius, who soon became emperor in Constantinople.