The Sack of Constantinople occurred from 8 to 13 April 1204 when a fleet of 22,000 French and Venetian crusaders and 60 war galleys under Boniface of Montferrat and Doge Enrico Dandolo laid siege to the Byzantine capital of Constantinople. The crusaders attacked the capital after the Byzantine emperor was unable to pay them the promised reward for their assistance in the previous year's siege, and they proceeded to sack the city and establish a new crusader state, the Latin Empire, in the Byzantine Empire's place.
In 1202, Pope Innocent III called for a "Fourth Crusade" to recapture Jerusalem from the Ayyubids. A fleet of French crusaders set sail for the Holy Land, and they enlisted the financial and naval support of the blind, 80-year-old Venetian doge Enrico Dandolo in their quest to retake Jerusalem in exchange for helping Venice to conquer the Croatian port city of Zara from Hungary. In January 1203, the Byzantine prince Alexius Angelus offered financial and military aid to the crusaders if they diverted to the Byzantine capital of Constantinople to depose the usurper Alexius III of Byzantium, who had overthrown and blinded his uncle Isaac II of Byzantium. The Crusaders were allured by the great wealth of the Byzantine capital, and they decided to accept Alexius' invitation. On 23 June 1203, ignoring their excommunication by the Pope for their attacks on fellow Christians, the crusaders landed at Constantinople, and they conquered the city in August 1203. In January 1204, however, the newly-installed "Alexius IV" was overthrown in a popular uprising, and Alexius V of Byzantium rose to the throne, reneging on Alexius IV's promise to reward the crusaders.
On 8 February 1204, Emperor Alexius V had Alexius IV executed, provoking the Venetians and crusaders into laying siege to Constantinople with the goal of sacking and conquering the city. On 9 April, the Crusader forces began an assault on the Golden Horn fortifications, but their first assault was repelled due to poor weather. On 12 April 1204, the weather changed, allowing for the crusaders to launch a more successful second assault. The Venetians scaled the walls from the sea and pushed into the city, massacring the brave Varangian Guard after extremely bloody fighting. The crusaders set fires in the Blachernae district with the goal of fending off Byzantine counterattacks, but the fires unintentionally spread, engulfing much of the city. Alexius V fled through the Polyandriou Gate as the Crusaders looted, terrorized, and vandalized Constantinople for three days, stealing bronze horses from the Hippodrome and taking them to St. Mark's Basilica in Venice. Thousands of civilians were killed, and even nuns were raped by the crusaders. 900,000 silver marks were looted from the city, and the sack and massacre of Constantinople avenged the 1182 Massacre of the Latins.
The Byzantine nobility fled their conquered capital and established rump states such as the Empire of Nicaea, the Empire of Trebizond, the Despotate of Epirus, and the Principality of Achaea, all of which claimed to be the legitimate successors of the Roman Empire. Meanwhile, the crusaders established their own states, including the Latin Empire of Constantinope and the Kingdom of Thessalonica in Greece. Until 1261, the Byzantine Empire was fragmented into Greek and Latin states during an era which came to be known as the Frankokratia.