Peter III of Aragon's envoy, William of Castelnou, started negotiations with the rebels' leaders in Palermo. Realizing that they could not resist without foreign support, they acknowledged Peter and Constance as their king and queen. They appointed envoys to accompany Castelnou to Collo where the Aragonese fleet was assembling. After a short hesitation, Peter decided to intervene on the rebels' behalf and sailed to Sicily. He was declared king of Sicily at Palermo on 4September. Thereafter two realms, each ruled by a monarch styled king (or queen) of Sicily, coexisted for more than a century, with Charles and his successors ruling in southern Italy (known as the Kingdom of Naples) while Peter and his descendants ruled the island of Sicily.