Genoa and Pisa became the only states with trading rights in the Black Sea. In the same century the Republic conquered many settlements in Crimea, where the Genoese colony of Caffa was established. The alliance with the restored Byzantine Empire increased the wealth and power of Genoa, and simultaneously decreased Venetian and Pisan commerce. The Byzantine Empire had granted the majority of free trading rights to Genoa. In 1282 Pisa tried to gain control of the commerce and administration of Corsica, after being called for support by the judge Sinucello who revolted against Genoa.

In August 1282, part of the Genoese fleet blockaded Pisan commerce near the river Arno. During 1283 both Genoa and Pisa made war preparations. Genoa built 120 galleys, 60 of which belonged to the Republic, while the other 60 galleys were rented to individuals. More than 15,000 mercenaries were hired as rowmen and soldiers. The Pisan fleet avoided combat, and tried to wear out the Genoese fleet during 1283. On August 5, 1284, in the naval Battle of Meloria the Genoese fleet, consisting of 93 ships led by Oberto Doria and Benedetto I Zaccaria, defeated the Pisan fleet, which consisted of 72 ships and was led by Albertino Morosini and Ugolino della Gherardesca. Genoa captured 30 Pisan ships, and sank seven. About 8,000 Pisans were killed during the battle, more than half of the Pisan troops, which were about 14,000. The defeat of Pisa, which never fully recovered as a maritime competitor, resulted in gain of control of the commerce of Corsica by Genoa. The Sardinian town of Sassari, which was under Pisan control, became a commune or self-styled "free municipality" which was controlled by Genoa. Control of Sardinia, however, did not pass permanently to Genoa: the Aragonese kings of Naples disputed control and did not secure it until the fifteenth century.