In 1309, he received Nocera and Isernia and exchanged Montescaglioso for Sorrento and Castellammare di Stabia. He began serving his brother, King Robert, against the Ghibellines of Uguccione della Faggiuola in Tuscany. He led 300 knights and was given the title of "Vicar of Tuscany, Lombardy, Romagna, the city of Bertinoro, and the city of Ferrara and Captain-General of the Guelph party of Tuscany".
He celebrated a jubilee at Siena and then moved on to Florence, where he stayed with the Mozzi family and the commune granted him a stipend of 4,000 florins. He took Arezzo on 29 September 1314, but Uguccione took Lucca. At Robert's request, Philip I of Taranto brought reinforcements north and met with initial successes, but at the Battle of Montecatini, the combined forces of the Guelphs were defeated by Uguccione and the Ghibellines in a skirmish on the plain. Peter and Philip retreated to Fucecchio and the former died en route, in the marsh, his body was never found.