Urban's successor, Pope Clement IV granted the Kingdom of Sicily to King Louis IX's ambitious younger brother, Charles I of Anjou in 1265. Charles invaded southern Italy and Manfred died fighting against the invaders in the Battle of Benevento on 26 February 1266. Charles took full control of Manfred's kingdom and revived his plans about an anti-Byzantine coalition. As negotiations about the church union had not yielded results, Pope Clement embraced Charles' idea. He summoned Charles, the titular Latin Emperor Baldwin II and William to Viterbo in Italy to discuss the conditions of an invasion of the Byzantine Empire by Charles. Charles and William reached an agreement after lengthy discussions. William swore fealty to Charles and promised not to "make enfeoffments which will remain in effect after" his death "beyond 14,000  of land". He consented to the marriage of his daughter, Isabella with Charles's younger son, Philip. Although his wife was pregnant, William acknowledged Charles and Charles's descendants as his heirs even if Isabella and Philip had no children and Anna gave birth to a son, only stipulating their son's right to hold one-fifth of Achaea in fief of the Angevins. As historian Peter Lock emphasizes, William's concessions "shows the straits to which the principality had been reduced by the Greek offensive". In return, Charles promised military support for the recovery of the territory lost to the Byzantines, but the details of his assistance remained unclarified. The treaty was confirmed by Baldwin II and ceremoniously ratified in the presence of the Pope, 14 cardinals and further ecclesiastic and secular dignitaries in the papal chamber in Viterbo on 24 May 1267. In a separate treaty, Baldwin II ceded all his suzerain rights over Achaea to Charles and his successors in return for Charles' promise to assist him to recover Constantinople within six or seven years.