In 1238, Pope Gregory IX formed an alliance between Genoa and Venice against the empire, and consequently against Pisa, too. One year later, he excommunicated Frederick II and called for an anti-Empire council to be held in Rome in 1241. On May 3, 1241, a combined fleet of Pisan and Sicilian ships, led by the emperor's son Enzo, attacked a Genoese convoy carrying prelates from northern Italy and France, next to the isle of Giglio (Battle of Giglio), in front of Tuscany; the Genoese lost 25 ships, while about a thousand sailors, two cardinals, and one bishop were taken prisoner. After this major victory, the council in Rome failed, but Pisa was excommunicated. This extreme measure was only removed in 1257. Anyway, the Tuscan city tried to take advantage of the favourable situation to conquer the Corsican city of Aleria and even lay siege to Genoa itself in 1243.