Battle of Pelagonia The Battle of Pelagonia took place in September 1259 , between the Empire of Nicaea and the Despotate of Epirus , Sicily 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 . The exact location remains unclear . It has been called also Battle of Kastoria because the three Byzantine sources ( Pachymeres , George Akropolites , Gregoras ) informs us that the Epirotic camp was firstly attacked there in a location called Boril ' s Wood ( Βορίλλα λόγγος ) . However since the warfare includes also a siege of Prilep it is justifiably called battle of Pelagonia . 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 Doukaina ( 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 Doukas , 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 , 2 , 000 Cumans , 300 Germans , 13 , 000 Hungarians , and 4 , 000 Serbs , and some Vlachs . There were supposedly 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 baron of Karytaina Geoffrey of Briel , one of the Frankish leaders , 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 ; according to George Pachymeres this is because Michael ' s illegitimate son John quarrelled with William . On the next day , the Frankish knights attacked the German mercenaries under the duke of Carinthia 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 Palaiologos 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 Akropolites , Nikephoros Gregoras , and George Sphrantzes .