The Fourth Crusade (12021204) was a Latin Christian armed expedition called by Pope Innocent III. The stated intent of the expedition was to recapture the Muslim-controlled city of Jerusalem, by first defeating the powerful Egyptian Ayyubid Sultanate, the strongest Muslim state of the time. However, a sequence of economic and political events culminated in the Crusader army's 1202 siege of Zara and the 1204 sack of Constantinople, the capital of the Greek Christian-controlled Byzantine Empire, rather than Egypt as originally planned. This led to the partitioning of the Byzantine Empire by the Crusaders.

In exchange for building a dedicated fleet and providing sea transport, the Republic of Venice instructed the Crusaders to pay them for the fleet. However, since not all Crusaders sailed from Venice, the Crusaders could not pay fully the price of the fleet. The Venetian Doge Enrico Dandolo hence asked that the Crusaders help them capture Zadar (or Zara), on the Adriatic Sea. This led in November 1202 to the siege and sack of Zara, the first attack against a Catholic city by a Catholic Crusader army. The city was then brought under Venetian control. When the Pope heard of this, he excommunicated the Crusader army.

In January 1203, en route to Jerusalem, the Crusader leadership entered into an agreement with the Byzantine prince Alexios Angelos to divert the Crusade to Constantinople and restore his deposed father Isaac II Angelos as emperor. The intent of the Crusaders was then to continue to Jerusalem with promised Byzantine financial and military aid. On 23 June 1203, the main Crusader army reached Constantinople, while other contingents (perhaps a majority of all crusaders) continued to Acre.

In August 1203, following the siege of Constantinople, Alexios was crowned co-emperor. However, in January 1204 he was deposed by a popular uprising. The Crusaders were no longer able to receive their promised payments from Alexios. Following the murder of Alexios on 8 February, the Crusaders decided on the outright conquest of the city. In April 1204, they captured and plundered the city's enormous wealth. Only a handful of the Crusaders continued to the Holy Land thereafter.

The conquest of Constantinople was followed by the fragmentation of the Byzantine Empire into three states centered in Nicaea, Trebizond and Epirus. The Crusaders then founded several new Crusader states, known as Frankokratia, in former Byzantine territory, largely hinged upon the Latin Empire of Constantinople. The presence of the Latin Crusader states almost immediately led to war with the Byzantine successor states and with the Bulgarian Empire. The Nicaean Empire eventually recovered Constantinople and restored the Byzantine Empire in 1261.

The Fourth Crusade is considered to have solidified the EastWest Schism. The crusade dealt an irrevocable blow to the Byzantine Empire, contributing to its decline and fall.

Constantinople (; Greek: Κωνσταντινούπολις Kōnstantinoupolis; Latin: Constantinopolis; Ottoman Turkish: قسطنطينيه‎, romanized: Ḳosṭanṭīnīye) was the capital of the Roman/Byzantine Empire (330–1204 and 1261–1453), the Latin Empire (1204–1261), and the Ottoman Empire (1453–1922). The capital then moved to Ankara following the Turkish War of Independence. Officially renamed Istanbul in 1930, the city is today the largest city and financial centre of the Republic of Turkey (1923–present). It remains the largest city in Europe.

In 324, the ancient city of Byzantium was renamed "New Rome" and declared the new capital of the Roman Empire by Emperor Constantine the Great, after whom it was renamed, and dedicated on 11 May 330. Constantinople is generally considered to be the center and the "cradle of Orthodox Christian civilization". From the mid-5th century to the early 13th century, Constantinople was the largest and wealthiest city in Europe. The city became famous for its architectural masterpieces, such as Hagia Sophia, the cathedral of the Eastern Orthodox Church, which served as the seat of the Ecumenical Patriarchate, the sacred Imperial Palace where the Emperors lived, the Galata Tower, the Hippodrome, the Golden Gate of the Land Walls, and opulent aristocratic palaces. The University of Constantinople was founded in the fifth century and contained artistic and literary treasures before it was sacked in 1204 and 1453, including its vast Imperial Library which contained the remnants of the Library of Alexandria and had 100,000 volumes. The city was the home of the Ecumenical Patriarch of Constantinople and guardian of Christendom's holiest relics such as the Crown of thorns and the True Cross.

Constantinople was famed for its massive and complex fortifications, which ranked among the most sophisticated defensive architecture of Antiquity. The Theodosian Walls consisted of a double wall lying about 2 kilometres (1.2 mi) to the west of the first wall and a moat with palisades in front. Constantinople's location between the Golden Horn and the Sea of Marmara reduced the land area that needed defensive walls. The city was built intentionally to rival Rome, and it was claimed that several elevations within its walls matched Rome's 'seven hills'. The impenetrable defenses enclosed magnificent palaces, domes, and towers, the result of prosperity Constantinople achieved as the gateway between two continents (Europe and Asia) and two seas (the Mediterranean and the Black Sea). Although besieged on numerous occasions by various armies, the defenses of Constantinople proved impenetrable for nearly nine hundred years.

In 1204, however, the armies of the Fourth Crusade took and devastated the city and, for several decades, its inhabitants resided under Latin occupation in a dwindling and depopulated city. In 1261 the Byzantine Emperor Michael VIII Palaiologos liberated the city, and after the restoration under the Palaiologos dynasty, enjoyed a partial recovery. With the advent of the Ottoman Empire in 1299, the Byzantine Empire began to lose territories and the city began to lose population. By the early 15th century, the Byzantine Empire was reduced to just Constantinople and its environs, along with Morea in Greece, making it an enclave inside the Ottoman Empire; after a 53-day siege the city eventually fell to the Ottomans, led by Sultan Mehmed II, on 29 May 1453, whereafter it replaced Edirne (Adrianople) as the new capital of the Ottoman Empire.