he stipulated a pact with Genoa, too, further weakening the Pisan presence in southern Italy. To counter the Genoese predominance in the southern Tyrrhenian Sea, Pisa strengthened its relationship with their traditional Spanish and French bases (Marseille, Narbonne, Barcelona, etc.) and tried to defy the Venetian rule of the Adriatic Sea. In 1180, the two cities agreed to a nonaggression treaty in the Tyrrhenian and the Adriatic, but the death of Emperor Manuel Comnenus in Constantinople changed the situation. Soon, attacks on Venetian convoys were made. Pisa signed trade and political pacts with Ancona, Pula, Zara, Split, and Brindisi; in 1195, a Pisan fleet reached Pola to defend its independence from Venice, but the Serenissima soon reconquered the rebel sea town. One year later, the two cities signed a peace treaty, which resulted in favourable conditions for Pisa, but in 1199, the Pisans violated it by blockading the port of Brindisi in Apulia. In the following naval battle, they were defeated by the Venetians. The war that followed ended in 1206 with a treaty in which Pisa gave up all its hopes to expand in the Adriatic, though it maintained the trading posts it had established in the area. From that point on, the two cities were united against the rising power of Genoa and sometimes collaborated to increase the trading benefits in Constantinople. 13th century In 1209 in Lerici, two councils for a final resolution of the rivalry with Genoa were held. A 20-year peace treaty was signed, but when in 1220, the emperor Frederick II confirmed his supremacy over the Tyrrhenian coast from Civitavecchia to Portovenere, the Genoese and Tuscan resentment against Pisa grew again. In the following years, Pisa clashed with Lucca in Garfagnana and was defeated by the Florentines at Castel del Bosco. The strong Ghibelline position of Pisa brought this town diametrically against the Pope, who was in a strong dispute with the Empire, and indeed the pope tried to deprive the town of its dominions in northern Sardinia. 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