In 1200, Pope Innocent III asked Europe's leaders to try again to capture Jerusalem - a task hopefully made easier by the death of the great Muslim leader Saladin. Two years later, the Crusaders who answered his call assembled in Venice. The city had agreed to ferry 30,000 soldiers and 4,500 horses to the Holy Land. However, the Crusaders didn't have enough cash to pay the bill, forcing the Venetians to do a deal. They offered cheap rides to Egypt if the warriors captured Zara, now Zadar in modern Croatia.
So the would-be liberators of Jerusalem got off to a flying start by attacking a Catholic city - one that just happened to be a commercial rival to the Pearl of the Adriatic.
Then the Venetians encouraged the Crusaders to turn their attention to Constantinople. Twice they attacked the civilised capital of Byzantium, with the alleged aims of restoring a deposed emperor, reuniting the divided Catholic church - and getting their bills paid.
On April 12, 1204, the Crusaders captured Constantinople, indulging in an orgy of looting and destruction. Drunk and disordered, they laid waste to monasteries, libraries and churches. They murdered the emperor and sat a prostitute on the bishop's throne, singing naughty French songs as they did so. Then they got down to business and divided up the Byzantine empire.
Venice got the best bits (no surprise there).
Historian Stephen Runciman describes the fourth crusade as a great crime against humanity: "It destroyed the treasures of the past, broke down the most advanced culture of Europe ... and implanted in the Greeks a hostility towards the west that has never entirely disappeared".
Oh, and the Crusaders never made it to Jerusalem either. In fact, many knights abandoned their outposts in the Holy Land to return home and grab their share of the spoils of Constantinople. Quite a result, really.