Pope Innocent III didn’t plan it that way, but the 4th Crusade (1202-1204) was mostly Christians fighting Christians.When the knights involved in that crusade decided to go to the Holy Land by sea, they contracted with the Venetians to carry 30,000 bodies and associated horses and supplies to retake Jerusalem.
There was only one little problem. Although the Venetians spent a year building a fleet to carry the 30,000 crusaders, only about a third of that number showed up. Oops! That didn’t sit well with Enrico Dandolo, the 90-year-old blind Doge of Venice—but no pushover when it came to negotiations.
Venice was upset that the Adriatic port of Zara (now called Zadar) now belonged to Hungary. The Doge negotiated with the leaders of the crusade to stop and capture Zara “on the way” to the East. Unfortunately, Zara was not only Christian: It was Roman Catholic. And King Emico of Hungary wore the crusaders’ cross himself. A deal was a deal, and the crusade did not want to start on in debt to the Venetians. So, they attacked and took Zara, returning it to the Venetians.
That was only the beginning of their problems. A Byzantine prince named Alexius Angelos offered to pay a fortune to the crusade and to their Venetian transport … if only they would see fit to returning him to the throne of Constantinople. His father, Isaac II Angelos, has been the emperor; but his elder brother Alexius III Angelos, had him blinded and deposed him.
It looked like a good deal. Although Prince Alexius had been drumming up support for his cause among the crowned heads of Europe, he was pretty much ignored. Too flighty, it seems. But the crusaders were committed, and the idea of all that loot turned their heads.
So off to Constantinople they sailed. They besieged the city from the Golden Horn side, and after a number of attacks finally prevailed. Prince Alexius was set up as Alexius IV Angelos. And now it was time for payback. Except, Alexius IV was unable or unwilling to pay what he had promised. So the crusaders not only took the city, but looted and burned it, raping and killing at will. And, um, they never did get to the Holy Land.
Pope Innocent III was furious. If you can’t trust 10,000 crusaders wearing crosses over their armor to do what they promised, whom can you trust?
The story is well told in Jonathan Phillips’s The Fourth Crusade and the Sack of Constantinople, complete with extensive notes, a large bibliography, and an alphabetical index.