With Anatolia secure, the Roman Emperor John III Doukas Vatatzes wanted to free Constantinople from the Latin occupiers who had stolen and destroyed their city. This was the driving mission of Romans in Anatolia, to liberate the Queen of Cities. In the 1230’s the Romans vanquished Latin power in Thrace, reducing them to little more than a city-state hiding behind the Theodosian walls. To try to dislodge them, John III engaged in an unlikely alliance with Bulgaria, and a joint siege of Constantinople was laid in 1235.
Bulgaria had emerged as a big power in the Balkans, for a brief period the largest power after defeating the Despotate of Epiros, their Roman rivals to the West. Perhaps those events led the Bulgarians to underestimate Nicaea. The Romans and Bulgarians agreed the Treaty of Kallipolis where they agreed to destroy the Latins and partition Thrace in a mutually beneficial manner. To cement the alliance, Theodore II Laskaris married Elena of Bulgaria. With the son of the Roman Emperor and the daughter of the Bulgarian Tsar Ivan Asen II in union, a fruitful alliance seemed in place. The first mission was to oust the crusaders from Constantinople.
Taking Constantinople was not easy though, and the Roman-Bulgarian army faced serious challenges. In 1235 the siege began. The Roman fleet attempted a blockade of Constantinople, the Bulgarians had no fleet – but both sides combined to besiege the city by land. The only way to take Constantinople was an attack by land AND sea, as the Fourth Crusade had shown.
The Pope was infuriated, and even was calling for a crusade against the Romans, urging Hungary to send an army to break the siege. It was not needed – the Romans failed to achieve naval superiority. The Venetian fleet along with Geoffrey de Villehardouin, who sent reinforcements to help the Latin occupiers Constantinople from the Morea, defeated the Roman fleet. This caused the siege to fail.
In the spring of 1236, the allies returned to attack. Yet again, they could not really make much progress. It is likely there was great distrust between the sides, how could the Romans trust the Bulgarians not to try to take Constantinople for their own empire? This distrust is illustrated by the sudden change in the balance of power – the Bulgarians under Ivan Asen decided to renege on the alliance and decided to ally with the Latins. Apprehensive about the growing strength of Nicaea, the Bulgarians no longer saw it as beneficial. In 1237, the Bulgarians and Latins attacked the the Roman fortress of Tzouroulos.
Constantinople had to wait for now, so after the collapse of the siege and the Bulgarian betrayal, the Emperor John III decided to focus on growing his power on Europe and launched operations into Macedonia to liberate territories there. The Latins knew how lucky they got, and Baldwin IV had to go to Europe and beg for a crusade to restore his failing state. In the aftermath of the siege, the Crown of Thorns, once among the most precious relics of the Roman Emperors in Constantinople was mortgaged to Venice to raise funds, and then sold to France. The Latin Empire was showing itself as unviable.
Sources:
Forty Sieges of Constantinople by John D. Grainger
The New Roman Empire by Anthony Kaldellis
The Byzantine Hellene by Dimiter Angelov
shadowsofconstantinople.com 