In the summer of 1242, a Mongol force invaded the Latin Empire of Constantinople. This force, a detachment of the army under Kadan then devastating Bulgaria, entered the empire from the north. It was met by the Emperor Baldwin II, who was victorious in a first encounter but was subsequently defeated.
The encounters probably took place in Thrace, but little can be said about them owing to the paucity of sources. Subsequent relations between Baldwin and the Mongol khans have been taken as evidence by some that Baldwin was captured and forced to make submission to the Mongols and pay tribute. Together with the major Mongol invasion of Anatolia the following year (1243), the Mongol defeat of Baldwin precipitated a power shift in the Aegean world.
Invasion
Baldwin II had made an alliance with some Cumans under their leaders Saronius and Jonas by 1239. It seems likely that he was giving shelter to Cumans fleeing the Mongols. The same act of giving shelter to the Mongols' Cuman enemies was the pretext for the Mongol invasion of Hungary, and probably also for the invasion of Bulgaria. It is likely that the attack on the Latin Empire resulted from the same motive: to punish the protectors of the Cumans.
Baldwin II was in Constantinople on 12 February 1242, when he addressed a letter to King Louis IX of France. He was again in Constantinople when he addressed a letter to Louis's influential mother, Blanche of Castile, on 5 August 1243. The Mongol invasion must have taken place between these dates, since it drew Baldwin away from the capital. The sources indicate only that the battles took place in Greece, a broad term in medieval sources, which could mean all the territory claimed by the Latin and Byzantine empires. It definitely included Thrace, which was part of the Latin Empire and bordered Bulgaria, which makes it the likely location of the Mongol raids.
According to the Chronicon Austriacum, Baldwin fought two battles with the invading force, which included some Cuman allies of the Mongols. Historians have offered several explanations of the Austrian chronicle's two battles and for Baldwin's motive in riding out to meet the invader. Peter Jackson suggests that Baldwin's initial victory may have come at the expense of these Cumans before the Mongol force arrived to defeat him. John Giebfried, on the other hand, suggests that the two battles may in fact be two phases of a single battle, making Baldwin II the victim of a feigned retreat. He argues, however, that Baldwin possessed sufficiently strong forces to have defeated a Mongol army. He had an alliance of his own with a group of Cumans and had recruited a large army in western Europe for his crusade against Tzurulon in 1239. Jean Richard suggests that in 1242 Baldwin may have been defending his Cuman allies when they came under Mongol attack. Henry Howorth suggests that he had been called to the defence of the young ruler of Bulgaria, Kaliman I, who was his vassal.
Baldwin may have been captured after his defeat, which would explain how rumours of his death originated. In that event, he was likely forced to accept Mongol overlordship and to make annual tribute payments in return for release.
Aftermath
By 1251 or 1252, Baldwin II certainly had diplomatic relations with the Mongol Empire, since he sent an ambassador, Baldwin of Hainaut, all the way to the imperial Mongol capital of Karakorum. In 1253, he gave William of Rubruck, a Franciscan missionary, letters of recommendation for Sartaq, the son of Batu, khan of the Golden Horde. Batu was Kadan's superior in 1242 and his army had also invaded Bulgaria.
Jean Richard suggests that Baldwin of Hainaut's mission was a renewal of submission, since a new khan had been elected since 1242. The Latin Empire is not listed by William of Rubruck among the tributaries of the Mongol Empire, however, nor was Baldwin II excommunicated for accepting Mongol overlordship as Bohemond V of Antioch was.
The Mongol invasion of the Latin Empire took place just a year before the Mongols' crushing victory over the Seljuks of Anatolia at the Battle of Köse Dağ (26 June 1243). Although Baldwin II had negotiated an alliance with the Seljuks in 1241, it was the Byzantine emperor John III Vatatzes who provided aid to the Seljuks, his erstwhile enemies, at a critical juncture in 1242 while they were under Mongol attack. As a result, the position of Vatatzes was strengthened with regards to the rump Seljuk state and the position of Baldwin, defeated by the Mongols himself, was weakened. Moreover, a two-year truce between Baldwin and Vatatzes expired on 24 June 1243. The Byzantine emperor seems to have taken advantage of Baldwin's situation to attempt to retake Constantinople, albeit unsuccessfully, as reported in the chronicle of Martin da Canal.