In 1468, after his father's death, he was sent to capture Rimini, which was held by Sigismondo's lover and later third wife Isotta degli Atti in the behalf of Sallustio, who was designated by his father as his heir. According to the tradition, he entered the "Rocca" ("Citadel") of the city disguised as a peasant, and took the city in the name of the Pope, but soon started to act independently. On 30 August 1469, thanks to the support of Federico da Montefeltro, he decisively defeated the Papal troops sent to oust him. In the following years, he is said to have poisoned both his stepmother, Isotta, and his half-brothers, Sallustio (whom he had initially associated in the government of Rimini) and Valerio. In 1475, he was finally invested with the vicariate of Rimini by the new pope, Sixtus IV; in the same year, he was married to Federico's daughter, Elisabetta. Hired by Florence after a period as Papal commander, he defeated the allied troops of Girolamo Riario near Perugia in 1479; he was a nephew of the pope who was trying to establish a lordship in Imola and Faenza, and the King of Naples. In the course of the so-called War of Ferrara, Malatesta, who had re-conciliated with the pope, fought as Venetian commander. His great achievement was the liberation of Rome by the victory of Campo Morto (21 August 1482), when, at the head of the Venetian-Papal forces, he completely defeated the royal army of Naples under the command of Duke Alfonso of Calabria.