Thessaly in the 1210s. They annihilated the Kingdom of Thessalonica and captured the city of Thessalonica in December 1224. Their expansion towards Constantinople came to an abrupt end when they suffered a massive defeat by the Bulgarians in the Battle of Klokotnitsa in 1230. The Greeks of Nicaea launched invasions against the Latin Empire from the east. By 1235, they reconquered Anatolia and seized a European bridgehead at Gallipoli. In theory, the princes of Achaea still owed allegiance to the Latin Emperors, but in practice Emperor Baldwin II would have been unable to defend Constantinople without Achaean military and financial support. In return, he acknowledged Geoffrey's elder son and successor, Geoffrey II, as the overlord of the triarchs (three co-rulers) of the island of Negroponte (and possibly of other Aegean islands). Early life William was the second son of Geoffrey I of Villehardouin and his wife Elisabeth. Elisabeth remained in France when her husband left for the Fourth Crusade. She and their elder son, Geoffrey, only moved to Achaea when Geoffrey's position stabilized after the Parliament of Ravennika. She gave birth to William in the castle of Kalamata around 1211. Growing up in the Morea, William could speak Greek like a native and he felt home with both Franks and Greeks. As a younger son, William received the Barony of Kalamata in fief, while his brother succeeded their father as prince around 1229. William married an unnamed daughter of Narjot de Toucy, a high-ranking official of the Latin Empire. He administered Achaea as regent during his brother's military campaigns for the defense of Constantinople. Reign Expansion and crusade William came to power in Achaea when the childless Geoffrey II died in the summer of 1246. By the time of William's ascension, the relationship between Epirus and Nicaea had grown tense. In December 1246, Nicaean troops attacked and captured Thessalonica. William took advantage of the two Greek powers' conflict to complete the conquest of the southeastern Morea. He laid siege to Monemvasia with the support of a Venetian fleet and other Frankish rulers, including Guy I de la Roche, Lord of Athens (who owed allegiance to William for the Moreot fiefs of Argos and Nauplia), and Angelo Sanudo, Duke of the Archipelago. The defenders suffered because of famine, but they capitulated only in 1248, after William promised to respect their property and liberty. He rewarded the town's leaders with estates and