The band of knights who gathered at Count Thibaut of Champaigne's castle in November 1199 intended simply to enjoy their host's hospitality and impress their ladies in jousting tournaments. But when the electric preacher Fulk of Neuilly gained entrance to the castle and publicly lamented the success of Saladin's Muslim forces in the Holy Land, frivolity left the hall. Wearing crosses of cloth across their shoulders, Count Thibaut and a company of knights marched to Pope Innocent III and pledged their lives to war.

Innocent was delighted with Thibaut's offer. For over a year, he'd worked among the monastic orders to inflame crusader wrath over the losses Saladin had inflicted—to little effect. Finally, a new leader had emerged to challenge the Muslims. Rome could now answer the frantic calls for help coming from besieged crusaders in the coastal city of Tyre.

Innocent and Thibaut decided to strike first at Egypt, the vulnerable underbelly of Saladin's forces. In 1201, Thibaut sent envoys to the doge of Venice requesting ships for the voyage, since the Turks now made land passage too dangerous. The doge agreed, setting a departure date of June 1202. But he charged a hefty price of 85,000 silver marks.

Deals in the dark

Then the unexpected happened. Thibaut fell ill and died, and another nobleman, Boniface of Montferrat, took his place as commander of the Fourth Crusade. While visiting a friend in Germany, Boniface met Alexius Angelinus, son of Constantinople's dispossessed emperor, now seeking men and arms in his quest to regain the throne of Christianity's eastern empire.

Alexius tried to persuade Boniface to redirect the crusaders away from Egypt and retake Constantinople for the Angelina family. In return, Alexius offered part of Constantinople and the Byzantine empire to Boniface as his personal fief. And, as a bone to the pope, he promised to enforce the Eastern Church's submission to Rome as the supreme head of the Church.

Returning to Rome, Boniface asked Innocent for permission to direct the crusade through Constantinople. The pope, concerned for East-West relations, said no.

When the time came for the crusaders to sail, the debt to Venice had not yet been fully paid. The doge insisted that Boniface could sail only if the army first captured Venice's rival Hungarian town of Zara. Boniface agreed, and Zara fell to the crusaders in November 1202. Innocent was aghast—he denounced the pillage of Zara and reproached Boniface for attacking fellow Christians.

But Innocent was no longer in control of his crusade. Boniface still owed the doge a substantial debt, and he decided to take Alexius up on his offer. The pope received word that Boniface was now taking the army to Constantinople. At first, he rationalized the move, hoping Alexius would help his cause in the Holy Land. Upon hearing that a crusader army was marching on the Greek capital, Constantinople's emperor fled his throne, and city officials installed Alexius in his stead.

The conflagration begins

All had gone according to plan, or so Alexius thought. But when the new emperor—keeping his promise to the papacy—commanded his Orthodox bishops to submit to Rome, he was met with sullen silence. Further, seeing the French swagger down their streets angered many Greeks. Their anger increased when Venetians acting out of "piety" set fire to a mosque visited by Muslim merchants, and it burned out of control, sweeping the city in a massive conflagration.

Soon a mob of Greeks was storming the palace, and one of the leaders—now declared the new emperor—threw Alexius into the imperial dungeon and strangled him.

Nothing remained for the crusaders but war. Frankish bishops denounced the new emperor as a murderer and the Greeks as schismatic, and declared that it was Rome's duty to subjugate Constantinople by force. After a failed first attempt, on April 12, 1204 the army attacked from their Venetian ships. Crossing over on ladders thrown from the ships' masts, the crusaders took the city's towers, and with the cry of "Holy Sepulchre!" they sent the Greek army into retreat. By the end of the day, Constantinople had fallen.

Massacre and mayhem

The starving, exhausted soldiers looted palaces and peasants alike, heedless of Boniface's attempts to control the pillaging.

Not even the hallowed Sancta Sophia was safe. Historian Donald Queller describes the scene: "Mules and other beasts of burden were driven up to the altar to bear away the precious ornaments of the church, desecrating it with their droppings." While soldiers drank from the altar-vessels, writes Steven Runciman, "a prostitute set herself on the Patriarch's throne and began to sing a ribald French song."

In the West, Innocent at first received the news of Constantinople's fall with enthusiasm. The pope thought he could now depend on the city to fight the Muslims. The Sultan of Egypt, he had heard, was already trembling, fearing he was next.

But when the details of crusader barbarism filtered back, Innocent was devastated. How could he speak of unity with his Orthodox brethren when such unspeakable acts had been committed against them by his own soldiers?

Innocent's regret came too late. He had hoped to make Constantinople his ally—instead, the Fourth Crusade sealed the schism between West and East. And it further weakened the city, now targeted by the rising power of the Ottoman Turks.