Fourth Crusade

The Fourth Crusade 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. However, a sequence of economic and political events culminated in the Crusader army's 1202 siege of Zara and the 1204 sack of Constantinople, rather than the conquest of Egypt as originally planned. This led to the partition of the Byzantine Empire by the Crusaders and their Venetian allies, leading to a period known as the Frankokratia.
In 1201, the Republic of Venice contracted with the Crusader leaders to build a dedicated fleet to transport their invasion force. However, the leaders greatly overestimated the number of soldiers who would embark from Venice, since many sailed from other ports, and the army that appeared could not pay the contracted price. In lieu of payment, the Venetian Doge Enrico Dandolo proposed that the Crusaders back him in attacking the rebellious city of Zara on the eastern Adriatic coast. This led in November 1202 to the siege and sack of Zara, the first attack against a Catholic city by a Catholic Crusader army, despite Pope Innocent III's calls for the Crusaders not to attack fellow Christians. The city was then brought under Venetian control. When the Pope heard of this, he temporarily 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 their main force to Constantinople and restore his deposed father Isaac II Angelos as emperor, who would then add his support to their invasion of Jerusalem. On 23 June 1203, the main Crusader army reached Constantinople, while other contingents 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, depriving the Crusaders of their promised bounty payments. 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. Several prominent Crusaders, including Enguerrand II of Boves, Simon de Montfort, 5th Earl of Leicester and Guy of Vaux-de-Cernay, among others, disagreed with the attacks on Zara and Constantinople, refused to take part in them and left the crusade.
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 Roman 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 July 1261.
The Fourth Crusade is considered to have solidified the East–West Schism. The crusade dealt an irrevocable blow to the Byzantine Empire, contributing to its decline and fall as all the unstable governments in the region, the Sack of Constantinople, and the thousands of deaths had left the region depleted of soldiers, resources, people and money, leaving it vulnerable to attack. Additionally, the empire had badly shrunk as it lost control of most of the Balkans, Anatolia, and Aegean islands. This made the restored empire both territorially diminished and vulnerable to invasions from the expanding Ottomans in the following centuries, to which the Byzantines ultimately succumbed in 1453.

Background

Loss of Jerusalem to the truce of 1198

In 1187, the Ayyubid Sultanate under Saladin conquered most of the Crusader states in the Levant. Jerusalem was lost to the Ayyubids as a result of the siege of Jerusalem in 1187 leading to the calling of the Third Crusade. The Crusader states were then reduced by Saladin to little more than three cities along the coast of the Mediterranean Sea: Tyre, Tripoli and Antioch.
The Third Crusade was launched in response to the fall of Jerusalem, with the goal of recovering the city. It successfully reclaimed an extensive territory, effectively reestablishing the Kingdom of Jerusalem. Although Jerusalem itself was not recovered, the important coastal towns of Acre and Jaffa were. On 2 September 1192, the Treaty of Jaffa was signed with Saladin, bringing the crusade to an end. The truce would last for three years and eight months.
The crusade had also been marked by a significant escalation in longstanding tensions between the feudal states of western Europe and the Byzantine Empire. During the crusade, Emperor Frederick I Barbarossa had almost besieged Constantinople because of the failure of the Byzantine government and Emperor, Isaac II Angelos, to provide him with safe passage across the Dardanelles because Isaac was busy fighting a pretender named Theodore Mangaphas. The Byzantines for their part suspected him of conspiring with the breakaway Byzantine provinces of Serbia and Bulgaria as Frederick Barbarossa was on friendly terms with Grand Prince Stefan Nemanja of Serbia and also got a letter getting support and fealty from Tsar Ivan Asen I of Bulgaria. King Richard I Lionheart of England also seized the breakaway Eastern Roman province of Cyprus. Rather than return it to the Empire, he gave the island to Guy of Lusignan, the former king of Jerusalem, who lost the crown to a former Eastern Roman ally, Conrad of Montferrat.
Saladin died on 4 March 1193, before the expiration of the truces, and his empire was contested and divided between three of his sons and two of his brothers. The new ruler of the Kingdom of Jerusalem, Henry II of Champagne, signed an extension of the truce with Egyptian Sultan al-Aziz Uthman. In 1197, the peace was interrupted by the arrival of the German Crusade of 1197. Without the permission of Henry, the Germans attacked the territory of al-Adil I of Damascus, who responded by attacking Jaffa. The sudden death of Henry prevented the relief of the port and the city was taken by force. The Germans did, however, succeed in capturing Beirut in the north.
Henry was succeeded by Aimery of Cyprus, who signed a truce with al-Adil of five years and eight months on 1 July 1198. The truce preserved the status quo: Jaffa remained in Ayyubid hands, but its destroyed fortifications could not be rebuilt; Beirut was left to the crusaders; and Sidon was placed under a revenue-sharing condominium. Before the expiration of the new truce on 1 March 1204, al-Adil succeeded in uniting the former empire of Saladin, acquiring Egypt in 1200 and Aleppo in 1202. As a result, his domains almost completely surrounded the diminished Crusader states.

Constantinople

Constantinople had been in existence for 874 years at the time of the Fourth Crusade and was the largest and most sophisticated city in Christendom. Almost alone amongst major medieval urban centres, it had retained the civic structures, public baths, forums, monuments, and aqueducts of classical Rome in working form. At its height, the city was home to an estimated population of about half a million people protected by 20 km of triple walls. Its planned location made Constantinople not only the capital of the surviving eastern part of the Roman Empire but also a commercial centre that dominated trade routes from the Mediterranean to the Black Sea, China, India and Persia. As a result, it was both a rival and a tempting target for the aggressive new states of the west, notably the Republic of Venice.
In 1195, the Byzantine Emperor Isaac II Angelos was deposed in favour of his brother by a palace coup. Ascending as Alexios III Angelos, the new emperor had his brother blinded and exiled. Ineffectual on the battlefield, Isaac had also proven to be an incompetent ruler who had let the treasury dwindle and outsourced the navy to the Venetians. His actions in wastefully distributing military weapons and supplies as gifts to his supporters had undermined the empire's defenses. The new emperor was to prove no better. Anxious to shore up his position, Alexios bankrupted the treasury. His attempts to secure the support of semi-autonomous border commanders undermined central authority. He neglected his crucial responsibilities for defence and diplomacy. The emperor's chief admiral, Michael Stryphnos, reportedly sold the fleet's equipment down to the very nails to enrich himself.

Rendezvous at Venice

succeeded to the papacy in January 1198, and the preaching of a new crusade became the prime goal of his pontificate, expounded in his bull Post miserabile. His call was largely ignored by the European monarchs: the Germans were struggling against Papal power, and England and France were still engaged in warfare against each other. However, due to the preaching of Fulk of Neuilly, in November Count Thibaut of Champagne hosted a knightly tournament at his castle in Écry-sur-Aisne. The event, though tinged with mourning for the recent death of Thibaut's uncle, King Richard I of England, became a turning point. During the festivities, Thibaut vowed to abandon martial games and instead dedicate his weapons to God's service, declaring his intention to join a crusade.
By early 1200, a council of leading crusader barons—including Thibaut, Louis of Blois, and Baldwin of Flanders—convened at Soissons to organize the expedition. They agreed to avoid the overland route through the Byzantine Empire and instead sail to Egypt, mirroring Richard I's strategy. To secure ships, the lords appointed a six-member committee, among them Geoffrey of Villehardouin, to negotiate contracts with maritime powers.
Thibaut was elected leader, but his sudden death in 1201 led to Boniface of Montferrat—brother of Conrad of Montferrat, who had died defending Tyre—assuming command. The committee ultimately secured a fleet through Venice.
Earlier crusades focused on the Land of Israel had involved the slow movement of large and disorganised land hosts across a generally hostile Anatolia. Egypt was now the dominant Muslim power in the eastern Mediterranean but also a major trading partner of Venice. An attack on Egypt would clearly be a maritime enterprise, requiring the creation of a fleet. Genoa was uninterested, but in March 1201 negotiations were opened with the doge of Venice, Enrico Dandolo who agreed to transport 33,500 crusaders, a very ambitious number. This was the moment, according to him, for the Venetian Republic to gain wealth, prestige, land, and trading routes in the Holy Land. This agreement required a full year of preparation on the part of the Venetians to build numerous ships and train the sailors who would man them, all the while curtailing the city's commercial activities. The crusading army was expected to consist of 4,500 knights, 9,000 squires, and 20,000 foot-soldiers. The majority of the crusading army that set out from Venice in early October 1202 originated from areas within France. It included men from Blois, Champagne, Amiens, Saint-Pol, the Île-de-France, and Burgundy. Several other regions of Europe sent substantial contingents as well, such as Flanders and Montferrat. Other notable groups came from the Holy Roman Empire, including the men under Martin, abbot of Pairis Abbey and Bishop Conrad of Halberstadt, together in alliance with the Venetian soldiers and sailors led by the doge, Enrico Dandolo. The crusade was to be ready to sail on 24 June 1203 and make directly for the Ayyubid capital, Cairo. This agreement was ratified by Pope Innocent, with a solemn ban on attacks on Christian states.