Despite his successes at Prinitza and Makryplagi, the war with the Byzantines had taken a toll on Achaean resources, and their empire remained a looming threat. A proposal to marry William's elder daughter Isabella to Andronikos, eldest son of Michael VIII, was strongly opposed by the Achaean nobility, who had no desire to come under Byzantine rule. Both William and his overlord Baldwin II, now dispossessed of Constantinople, had hoped for aid from King Manfred of Sicily, who had sent troops to aid William at Pelagonia. But Manfred fell under Papal sanction and was killed in 1266, when Charles of Anjou conquered his kingdom. Charles was now ascendant in Italy, and William and Baldwin came to terms with him in the Treaty of Viterbo (1267). In return for the military aid and funds they so greatly needed, Charles obtained the suzerainty over Achaea from Baldwin, and the Principality itself from William. The latter was to retain the Principality for life, and it was to pass to his daughter, Isabella, who was to marry one of Charles' sons.