in late 1261. The possession of the three forts facilitated further Byzantine expansion, and William was forced to seek external support. With the approval of Baldwin II, he swore fealty to the Angevin king of Sicily, Charles I. William acknowledged Charles and his descendants as his heirs in the Treaty of Viterbo on 24 May 1267. Charles sent troops to Achaea and with their help, William was able to resist Byzantine invasions during the last years of his reign. Background Ruling from 1246 to 1278, William was the fourth prince of Achaea. A state in Frankish Greece, Achaea was established in Byzantine territories in the Morea (in southern Greece) in the aftermath of the Fourth Crusade. William's father Geoffrey I of Villehardouin began the conquest of the Morea with a local Greek aristocrat's support in late 1204. As his ally soon died, Geoffrey approached Boniface of Montferrat, the ruler of the newly established Frankish Kingdom of Thessalonica, for military aid. Boniface appointed William of Champlitte to accompany Geoffrey back to the Morea and they mustered 500 knights and for the campaign. They captured Patras and other Byzantine fortresses and forced the Messenian and Arcadian Greek aristocrats into submission. William of Champlitte received the title of Prince of Achaea from Pope Innocent III in November 1205. The Franks could not capture Monemvasia in southeastern Morea, and Greek corsairs seized the fortress. The Frankish conquerors were also unable to overcome the natives in the southeastern mountainous region. In the newly established principality, Geoffrey held the Barony of Kalamata in Messenia in fief. When William died in 1208, Geoffrey assumed power likely with his peers' consent. He swore fealty to the Latin Emperor of Constantinople, Henry, at the Parliament of Ravennika in 1209. Henry confirmed Geoffrey as the new Prince of Achaea and made him an immediate imperial vassal. The leaders of the Fourth Crusade had promised much of the Morea to Venice in their 1204 treaty about the distribution of the Byzantine Empire. In a new treaty Geoffrey recognized he held parts of his principality in fief from Venice, but the Venetians took no further steps to assert their theoretical suzerainty. As the Franks could not conquer all the Byzantine territories, two successor states, Epirus and Nicaea, emerged on the western and eastern borderlands of the Byzantine Empire respectively. The Greeks of Epirus started the reconquest of the former Byzantine territories in