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The 'Battle of Pelagonia' took place in September of 1259, between the Empire of Nicaea and the Principality of Achaea. It was a decisive event in the Near East history, ensuring the Byzantine reconquest of Constantinople and the end of the Latin Empire in 1261, and marks the beginning of the Byzantine recovery of Greece.
Nicaean emperor Theodore II Laskaris died in 1258 and was succeeded by the young John IV Laskaris, under the regency of Michael VIII Palaiologos, who was determined to restore the Byzantine Empire and recapture all of the territory it held before the Fourth Crusade. In 1259 William II Villehardouin married Anna Komnena Dukaina (also known as Agnes), daughter of Michael II of Epirus, cementing an alliance between the Despotate of Epirus and Achaea against Nicaea. They also allied with Manfred of Sicily who sent them 400 knights.
In 1259 the Nicaeans invaded Thessaly and in September the Achaean and Epirote army marched north to meet them. The Nicaeans were led by the ''sebastocrator'' Theodore Ducas, the brother of Michael II of Epirus. According to the French ''Chronicle of Morea'', The Nicaean force consisted of the main Byzantine army, with Turkish mercenaries, 2000 Cumans, 300 Germans, 13 000 Hungarians, 4000 Serbs and Bulgarians, and some Vlachs. There were suppposedly 27 cavalry divisions, although all of these numbers are probably exaggerated. Theodore also gathered all the local peasants and their flocks and placed them on the hilltops, so that from far away they might appear to be part of the army.
Theodore then sent a false deserter to Michael II and William, exaggerating the number of Nicaean troops and chastizing Michael for attempting to attack a family member. The duke of Carinthia, who also had 300 Germans with him, did not believe the deserter, and convinced the Achaeans to stay when they decided to flee. Still, Michael and his troops deserted during the night and fled to the Nicaean side; according to George Pachymeres this is because Michael's illegitimate son John quarrelled with William.
On the next day, the Germans under the duke of Carinthia attacked their fellow German mercenaries on the Nicaean side. The duke was killed in the fight. The Hungarian archers then killed all the Achaean horses, leaving the knights effectively defenceless. The Achaean foot soldiers fled and the knights surrendered; prince William fled as well and hid under a nearby haystack where he was soon captured. Theodore brought him to John Palaiologos, brother of Michael VIII, who was in command of the expedition, and William was forced to give up strategic fortresses in Achaea (including Mystras) before he was set free.
John Palaeologus went on to capture Thebes. The Principality of Achaea, which had become the strongest French state in Greece in the aftermath of the Fourth Crusade, was now reduced to Nicaean vassalage; the Duchy of Athens soon became the dominant French state. Michael VIII took advantage of the defeat to recapture Constantinople in 1261.
There is a problem with the Chronicle of Morea's claim that the "duke of Carinthia" was present at the battle. The duke at the time was Ulrich III, but he ruled for many years after 1259, and was probably not at the battle; the writer of the Chronicle may have invented a fictitious duke as a counterbalance to William. Greek sources, aside from George Pachymeres, include George Acropolites, Nicephorus Gregoras, and George Sphranztes.