Pope Innocent III was the first pope since Urban II to be both eager and able to make the Crusade a major papal concern. In 1198 he called a new Crusade through legates and encyclical letters. In 1199 a tax was levied on all clerical incomes—later to become a precedent for systematic papal income taxes—and Fulk of Neuilly, a popular orator, was commissioned to preach. At a tournament held by Thibaut III of Champagne, several prominent French nobles took the cross. Among them was Geoffrey of Villehardouin, author of one of the principal accounts of the Crusade; other important nobles joined later, and contact was made with Venice to provide transport.

Unfortunately, Thibaut of Champagne died before the Crusaders departed for Venice, and the barons turned to Boniface of Montferrat, whose involvement as leader of the Crusade proved to be fateful. He had close family ties with both the Byzantine Empire and the Crusader states. His brother, Conrad of Montferrat, had received the crown of Jerusalem only to be killed by members of the Nizārī Ismāʿīliyyah shortly thereafter. Before going to the Holy Land, Conrad had married the sister of Emperor Isaac II Angelus and received the title of Caesar. Boniface was also the vassal of Philip of Swabia, who was a contender for the German throne and the son-in-law of Isaac II. In 1195 Isaac was blinded and deposed by his brother, who took the throne as Alexius III. Several years later Isaac’s son, also named Alexius, escaped from Constantinople and fled to Philip’s court. At Christmas 1201 Boniface, Philip, and the young Alexius discussed the possibility of using the Crusade to depose Alexius III and place the young man on the throne. Boniface sought the approval of the pope for the diversion, but Innocent refused to allow it. The young Alexius also journeyed to Rome but had no better luck with Innocent III. Despite the papal prohibition, Boniface and the Byzantine prince still hoped to find a way to move the Crusade toward Constantinople on its way to the Holy Land.

When the Crusade army arrived in Venice in the summer of 1202, it was only one-third of its projected size. This was a serious problem, since the French had contracted with the Venetians for a fleet and provisions that they now realized they neither needed nor could afford. The Venetians had incurred enormous expense for the French and were understandably upset by their inability to pay. The leader of Venice, Doge Enrico Dandolo, was a man of great sagacity and prudence who was in his 90s and completely blind by the time of the Crusade. Dandolo proposed that if the French would assist the Venetians in capturing the rebellious city of Zadar (now in Croatia), he would be willing to suspend the outstanding debt until it could be paid in captured booty. With few options, the Crusaders agreed, even though Zadar was a Christian city under the control of the king of Hungary, who had taken the Crusader’s vow. Innocent was informed of the plan, but his veto was disregarded. In November 1202 the Crusaders captured Zadar and wintered there. Reluctant to jeopardize the Crusade, Innocent gave conditional absolution to the Crusaders, but not to the Venetians.

Meanwhile, envoys from Philip of Swabia arrived at Zadar with an offer from Alexius, the Byzantine prince. If the Crusaders would sail to Constantinople and topple the reigning emperor, Alexius would place the Byzantine church in submission to Rome, pay the Crusaders an enormous sum, and join the Crusade to Egypt (now the centre of Muslim power in the Levant) with a large army. It was a tempting offer for an enterprise that was short on funds. The Crusade leaders accepted it, but a great many of the rank and file wanted nothing to do with the proposal, and many deserted. The Crusade sailed to Corfu before arriving in Constantinople in late June 1203. After the Crusaders attacked the northeastern corner of the city and then set a destructive fire, the citizens of Constantinople turned against Alexius III, who then fled. The Byzantine prince was elevated to the throne as Alexius IV along with his blind father, Isaac II.

Although the new emperor tried to make good his promises to the Crusaders, he soon ran short of money. He also faced anti-Latin hatred in Constantinople, which had been endemic for decades and now reached a fever pitch. Alexius IV, who owed his throne to Latins, became bitterly unpopular and was finally toppled in a palace coup in late January 1204. The Crusaders, now cheated of their reward and disgusted at the treachery of the Byzantines, declared war on Constantinople, which fell to the Fourth Crusade on April 12, 1204. What followed was one of the most profitable and disgraceful sacks of a city in history. Despite their oaths and the threat of excommunication, the Crusaders ruthlessly and systematically violated the city’s holy sanctuaries, destroying, defiling, or stealing all they could lay hands on. Many also broke their vows to respect the women of Constantinople and assaulted them. When Innocent III heard of the conduct of his pilgrims, he was filled with shame and strongly rebuked them.

Before the capture of the city, the Crusaders had decided that 12 electors (6 Venetians and 6 Franks) should choose an emperor who would rule one-fourth of the imperial domain. The other three-fourths was to be divided. The clergy of the party that did not include the emperor-elect were to oversee Hagia Sophia and choose a patriarch. A small amount of property was specifically designated to support the clergy, and the rest was divided as booty.

Once order had been restored, the Franks and the Venetians implemented their agreement; Baldwin of Flanders was elected emperor, and the Venetian Thomas Morosini was chosen patriarch. Various Latin-French lordships throughout Greece—in particular, the duchy of Athens and the principality of the Morea—did provide cultural contacts with western Europe and promoted the study of Greek. There was also a French impact on Greece. Notably, a collection of laws, the Assises de Romanie (Assizes of Romania), was produced. The Chronicle of the Morea appeared in both French and Greek (and later Aragonese) versions. Impressive remains of Crusader castles and Gothic churches can still be seen in Greece. Nevertheless, the Latin empire always rested on shaky foundations. Indeed, not all the Byzantine Empire was conquered by the Crusade. The imperial government continued in Nicaea, and the offshoot empire of Trebizond, at the eastern end of the Black Sea, lasted until 1461. The Byzantine despotate of Epirus was also established, and the Bulgarians remained hostile to the Crusaders. Finally, in 1261 a sadly diminished Constantinople was reconquered by Michael VIII Palaeologus with the aid of Genoa, the traditional rival of Venice. The city, however, would never be the same. For the remainder of its Christian history, it would remain poor, dilapidated, and largely abandoned.

The belief that the conquest of Constantinople would help Crusading efforts was a mirage. Indeed, the opposite was true, for the unstable Latin empire siphoned off much of Europe’s Crusading energy. The legacy of the Fourth Crusade was the deep sense of betrayal the Latins had instilled in their Greek coreligionists. With the events of 1204, the schism between the Catholic West and Orthodox East was complete.