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Pelavicino supported Frederick II against pope Gregory IX since 1234. Starting in 1250, he subdued the cities of Parma, Cremona, Piacenza, Pavia, and Brescia. Due to jealousness at Ezzelino III da Romano, Pelavicino joined the party of the Guelphs and took a great part in the victory of the Lombardic-Guelphish League of towns over Ezzelino at the Battle of Cassano. For this, he was awarded with the cities of Milano, Como, Lodi, Novara, Tortona, and Alessandria.