The sack of Constantinople occurred in April 1204 and marked the culmination of the Fourth Crusade. Crusader armies captured, looted, and destroyed parts of Constantinople, then the capital of the Byzantine Empire. After the capture of the city, the Latin Empire (known to the Byzantines as the Frankokratia or the Latin occupation) was established and Baldwin of Flanders was crowned Emperor Baldwin I of Constantinople in the Hagia Sophia.
After the city's sacking, most of the Byzantine Empire's territories were divided up among the Crusaders. Byzantine aristocrats also established a number of small independent splinter states—one of them being the Empire of Nicaea, which would eventually recapture Constantinople in 1261 and proclaim the reinstatement of the Empire. However, the restored Empire never managed to reclaim its former territorial or economic strength, and eventually fell to the rising Ottoman Empire in the 1453 Siege of Constantinople.
The Byzantine Empire was left much poorer, smaller, and ultimately less able to defend itself against the Seljuk and Ottoman conquests that followed; the actions of the Crusaders thus directly accelerated the collapse of Christendom in the east, and in the long run helped facilitate the later Ottoman conquests of Southeastern Europe.
The sack of Constantinople is a major turning point in medieval history. The Crusaders' decision to attack the world's largest Christian city was unprecedented and immediately controversial. Reports of Crusader looting and brutality scandalized and horrified the Orthodox world; relations between the Catholic and Orthodox churches were catastrophically wounded for many centuries afterwards, and would not be substantially repaired until modern times.
Before the siege
The Massacre of the Latins (; ), a massacre of the Roman Catholic or "Latin" inhabitants of Constantinople by the usurper Andronikos Komnenos and his supporters in May 1182, affected political relations between Western Europe and the Byzantine Empire and led to the sack of Thessalonica by Normans. Although regular trade agreements were soon resumed between Byzantine and Latin States, some Westerners sought some form of revenge.
Following the siege of Constantinople in 1203, on 1 August 1203 the pro-Crusader Alexios Angelos was crowned Emperor Alexios IV of the Byzantine Empire. He attempted to pacify the city, but riots between anti-Crusader Greeks and pro-Crusader Latins broke out later that month and lasted until November, during which time most of the populace began to turn against him.
On 25 January 1204, the death of co-Emperor Isaac II set off rioting in Constantinople in which the people deposed Alexios IV. He turned to the Crusaders for help, but was imprisoned by the imperial chamberlain, Alexios Doukas, who declared himself Emperor on 5 February before executing Alexios IV on 8 February by strangulation. Emperor Alexios V then attempted to negotiate with the Crusaders for a withdrawal from Byzantine territory without payment, but they refused in order to avenge Alexios IV and receive the money that was promised. In March 1204, the Crusader and Venetian leadership decided on the outright conquest of Constantinople in order to settle debts, and drew up a formal agreement to divide the Byzantine Empire between them.
Siege
By the end of March, the combined Crusader armies were besieging Constantinople as Emperor Alexios V began to strengthen the city's defences while conducting more active operations outside the city. By the first week of April, the Crusaders had begun their siege from their encampment in the town of Galata across the Golden Horn from Constantinople.
On 9 April 1204, the Crusader and Venetian forces began an assault on the Golden Horn fortifications by crossing the waterway to the northwest wall of the city, but, because of bad weather, the assault forces were driven back when the troops that landed came under heavy archery fire in open ground between Constantinople's fortifications and the shore.