The Fourth Crusade set out in 1202 to attack Egypt, could not pay the Venetians for its fleet, and ended by sacking Constantinople in 1204 without ever reaching the Holy Land.
The Fourth Crusade is the one that went wrong in a way that is still argued about eight centuries later, and the argument matters because the two available explanations point in very different directions.
The events are not in dispute. Innocent III proclaimed a crusade against Egypt in 1198. The army contracted with Venice for transport, could not pay, worked off part of the debt by capturing the Christian city of Zadar for Venice, was excommunicated for it, then diverted to Constantinople to install a claimant who had promised money it also never received, and finally stormed and sacked the city in April 1204.
What is disputed is whether that sequence was a conspiracy or an accident. The archive's position is stated in this article: it looks much more like a chain of contingent decisions, each defensible in isolation, made by men progressively trapped by a debt.
The contract with Venice was for a fleet to carry 33,500 men. About a third of that number arrived, because many crusaders had made their own arrangements from other ports, and the army could not pay a bill calculated for three times its size.
Venice had spent a year building the fleet and had suspended its other commerce to do it. Enrico Dandolo, the doge — old, reportedly blind, and one of the shrewdest political operators of the century — proposed that the debt be worked off by services. The first service was the capture of Zadar, a Christian city under Hungarian protection, in November 1202. Innocent III excommunicated the whole army for it.
At Zadar the crusade was approached by Alexios Angelos, son of the deposed emperor Isaac II, who offered enormous sums, ten thousand troops and the submission of the Greek church to Rome if the army would restore his father. It was a way out of a debt the crusade could not otherwise clear, and they took it.
The conspiracy readings come in several forms — that Venice planned the diversion to remove a commercial rival, that Philip of Swabia orchestrated it for his brother-in-law Alexios, that Boniface of Montferrat did.
The evidence for all of them is thin and mostly consists of the outcome. Against them: Venice had extensive privileges in Constantinople already and its trade depended on the empire functioning; the fleet was built to a contract that predated Alexios's approach; and the decision to storm the city in April 1204 was taken only after the regime the crusaders had installed collapsed and the money stopped.
The alternative reading is less satisfying and better supported. A debt too large to pay, an army too small to leave, a fleet it did not control, an offer that solved everything, a puppet who could not deliver, and a city that would not pay — with each step following from the last. The archive holds to that, while noting that Venice profited enormously and that the men who benefited most were the ones making the decisions.
After the city fell the victors partitioned the empire by a treaty drafted in advance — the Partitio Romaniae. Baldwin of Flanders became Latin emperor; Boniface of Montferrat took Thessalonica; Venice took three-eighths of the empire, chosen with an eye to harbours rather than territory, and the islands and ports that made it a maritime power for the next three centuries.
The Greek response was to reconstitute the empire elsewhere: at Nicaea, in Epirus, and at Trebizond, each claiming to be the real one. It was Nicaea that eventually won, and it took Constantinople back in 1261.
The crusade itself simply ended. No part of it went on to Egypt or Jerusalem, which had been the point.
The Latin Empire lasted fifty-seven years and was weak for all of them. The Byzantine Empire restored in 1261 held a fraction of its former territory, had lost its fleet and its treasury, and was permanently dependent on Italian commercial powers it could not tax.
The schism between the Greek and Latin churches, which had been a formal quarrel of theologians since 1054, became something a Greek population had personal reasons to hold. That the empire's later appeals for western help against the Ottomans went largely unanswered, and were bitterly resented at home when they were made, has its root here.
Innocent III, who had excommunicated the army and then had to accept what it did, wrote afterwards that the crusaders had turned their swords against Christians and taken the empire not for Christ but for themselves. It remains the sharpest verdict, and it was written by the man who called the crusade.