Guido came to his father's aid during the Rossi War against Ludovico il Moro and was thus declared a rebel by the duchy of Milan. He successfully held the Rocca dei Rossi in San Secondo against a Sforza siege under Antonio del Vasto (son of the marquess of Saluzzo), who was killed during the siege. Guido put a price on Sforza's head but the situation soon worsened. He was promised Venetian reinforcements but these were tied down in the War of Ferrara and could not reach him. Pier Maria II moved from San Secondo to Torrechiara, dying in the latter location on 1 September 1482. Some castles were betrayed, such as those at Carona, Bosco di Corniglio and Roccaferrara. San Secondo was still under siege - Guido attempted a night sortie on 12 September, but this failed.