Siege of Constantinople (1235–1236)

The siege of Constantinople (1235–1236) took place at the capital of the Latin Empire under Latin Emperor John of Brienne by a joint Bulgarian–Nicaean army led by the Tsar Ivan Asen II of Bulgaria and the Emperor of Nicaea John III Doukas Vatatzes.

The siege arose from the prolonged conflict between the Latin Empire, Bulgaria's ambitions, and the Byzantine successor states, i.e., Empire of Nicaea and Despotate of Epirus, following the Fourth Crusade's capture and sack of the city in 1204. The defeat of Epirus at Battle of Klokotnitsa in 1230 by Bulgarians led Vatatzes and Asen to negotiate and eventually agree to a formal alliance in early 1235, sealed by ecclesiastical concessions and a dynastic marriage. This alliance was aimed to combine their forces to reclaim Constantinople.

Despite their combined strength against a small garrison, the siege failed to breach Constantinople's fortifications supported by the Venetian navy. The allied army withdrew in the autumn for the winter and renewed the siege in 1236. Later that year Asen shifted his allegiance, presumably to avoid calls for a crusade from the pope or because sought to capture the city himself, before ultimately abandoning the Latin Empire a year later and refraining from future campaigns. While the siege did not topple the Latin Empire, it increased pressure against Latin‐rule and foreshadowed the eventual Byzantine recovery of Constantinople in 1261 by the Nicaean forces.

The Fourth Crusade resulted in the partition of Byzantine empire's territories among the crusaders. Starting in 1204, the newly established Latin Empire struggled to consolidate their power,^[3] as its authority was immediately challenged by Byzantine rump states (Empire of Nicaea and Despotate of Epirus). Meanwhile, in the north, Bulgarian gathered an army aimed to control parts of Macedonia and Thrace, regions claimed by the newly established Latin Empire.^[4]

The struggle for control of Macedonia and Thrace between the different regional powers, often forming and breaking agreements between them, led much destruction to cities of northern Greece, such as Serres and Philippopolis, and the displaced population sought refuge to well-fortified cities.^[5] A key city for the conquest of Constantinople was the Thessalonica, which was under the control of Theodore Doukas, ruler of Epirus, since 1224.^[6] The Epirote defeat, including the capture of Theodore Doukas, by the Bulgarians at the Battle of Klokotnitsa (1230) ended the Epirote aspirations.^[7][8]

The Latins had been negotiating with Asen, who was under the impression that he could become the Emperor of Constantinople, aimed at avoiding an Asen–Theodore Doukas alliance. Concurrently, the Latin barons and papacy at Perugia offered the throne of Constantinople to John of Brienne after the death of Robert of Courtenay in 1228, but this agreement was kept secret from non-Latin actors.^[9] John accepted this offer in 1229 and he arrived at Constantinople in 1231 for the coronation. The Latins also broke negotiations with Asen once Epirus was defeated by the Bulgarians in 1230. Asen was upset upon these news and renewed the communication with Nicaea for an alliance against the Latin Empire.^[10]

In the spring of 1235, Vatatzes crossed the Dardanelles and took over Gallipoli. Asen along with his family arrived there to meet with Vatatzes and his court, where they concluded an alliance for a joint campaign against the Latin Empire. Part of the agreement for the alliance was the recognition of the Bulgarian Church's autocephaly by the Nicaean church, granting the title of Patriarch of Bulgaria to the archbishop of Tărnovo. In return, Nicaean patriarch was recognized by Asen as ecumenical and he gave up his claims over Mount Athos, along with ecclesiastical authority in the areas of eastern Thrace and Thessaloniki.^[11] The alliance was sealed with the marriage between Asen's daughter, Elena, and the Nicean heir, Theodore. The joint campaign started the summer of the same year right after the marriage.^[12][13]

The Nicaean army moved from Gallipoli north, seizing territories from Latin Empire in Thrace from the east coast up to the mouth of the Marica, including the fortress of Tzurulum located west of Constantinople. Having secured the surrounding territory, the two armies joined forces to lay siege to Constantinople.^[14]

The Latin Emperor had at his command a small garrison, which included 160–800 knights.^[c] The Latins distrusted the Greeks living within the city and disarmed them. To avoid any uprising by the Greek population, they claimed that the Nicaean ships carried Turkish mercenaries whose purpose was to burn Constantinople.^[15]

Both Asen and Vatatzes personally participated in the combat.^[16] The defenders were able to repel the besieging forces in land and on sea. On land, John of Brienne used his knights to charge against Asen–Vatatzes army outside the walls of the city causing casualties and confusion to the allied army.^[15] The Nicaean fleet tried to penetrate the defenses of the Golden Horn but it was initially repelled by the defenders. Upon the arrival of reinforcement of a naval squadron (25 armed galleys) from Republic of Venice for the defense of Constantinople, the Venetians pursued the retreating Nicaean ships and they were able to capture 24 of them.^[15] The joint Bulgarian–Nicaean siege was unsuccessful and they retreated in the autumn due to the incoming winter. Asen and Vatatzes agreed to continue the siege in the following year and after the departure of the Venetian fleet.^[17]

The second phase of the siege, lasting into the spring of 1236, brought no significant progress. The defenders had received assistance from Geoffrey II Villehardouin of Morea and the Italian republics (Venice, Genoa and Pisa).^[14][17] Historian John Langdon argued that Asen and Vatatzes met in Thrace to organize disopition of forces and reinforce any Thracian holdings to prevent further reinforcements arriving to Constantinople after which they departed to their courts.^[18]

The alliance between Asen and Vatatzes was short-lived and instead Asen allied with the Latin Empire. Asen's army reinforced by displaced Cumans from the Steppe by the Mongols attacked the Nicaean possessions, including the fortress of Tzurulum.^[19] Scholars have presented two plausible explanations for this action. He may had reconsidered the agreement upon realizing that Nicaea stood to gain the most. If Constantinople was captured, it would had become the center of a Greek Empire preventing in the future its capture by Bulgarians.^[14] At the same time, Pope Gregory IX increased calls for a crusade against him, particularly urging Hungary to attack from the north, because Asen refused to accept union with the Church of Rome and instead he allied with schismatic opponents against Catholic-held territories. Asen managed to avert this threat through diplomatic outreach to Hungary and Constantinople and by refraining from further attacks.^[20][21]

Asen broke his alliance with the Latin Empire by the end of 1237 and made a treaty with Vatatzes. According to contemporary historian, George Akropolites, this reversal was prompted by the sudden death of his wife and one of his children from a plague outbreak, which was interpreted as divine punishment for breaking his previous alliance.^[11]

In the years that followed, Asen focused on consolidating Bulgaria's defenses and improving relations with Hungary. The kingdom faced threats not only from a potential crusade but also from the northeast, where the Mongols and Tatars had destroyed the Cuman state. This new danger imperiled both Bulgaria and Hungary and effectively halted any potential expansions to the south.^[22]

After the dissolution of the besieging army, some of John of Brienne's soldiers left his service because they were not paid, further weakening the defenses of the city.^[23] In 1237, the emperor recruited Western knights for the defense of the city in his trip west with the help of the pope.^[20] Two years later, more knights (estimated 30,000 to 60,000 in number) were mobilized. They crossed Hungary and Bulgaria, where Asen not wanting to fight them gave them free passage. They captured the Nicaean fortress of Tzurulum before dispersing, many returning home and others joining in the defense of Constantinople.^[24] The sporadic reinforcements were insufficient to counter the pressure from Nicaean and Bulgarian campaigns, the capital of the Latin Empire was spared only because of the divisions among its enemies.^[14]

Vatatzes expanded his holdings in Thrace during the campaign and a year after Asen's death in 1241, he was able to secure Thessalonica, isolating Constantinople.^[25] At the same time, Vatatzes had to return to defend his homeland in Anatolia against a possible Mongol invasion. Nicaea was spared, because the Mongols retreated following the sudden death of Great Khan in Karakorum. It was a boon for Nicaea because Mongols had inflicted great damage to the Seljuk Empire,^[26] leaving Nicaea in a stronger position to claim Constantinople.^[27]

In 1258, the Battle of Pelagonia signaled the beginning of the end of Latin predominance in Greece. On 25 July 1261, with most of the Latin troops away on campaign, the Nicaean general Alexios Strategopoulos found an unguarded entrance to the city, and entered it with his troops, restoring the Byzantine Empire under Michael VIII Palaiologos.^[28]

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