The following year he sided with Braccio da Montone, who was leading a party of exiles from Perugia, in the attempt to retake Mantua, which was now under Biordo dei Michelotti; Biordo hired him for some operations in the Val Nocera.
At the battle of Sant'Egidio (12 July 1416, later portrayed in a famous fresco by Paolo Uccello), the bracceschi were victorious, and Perugia was finally forced to open its gates to Braccio da Montone.
Guidantonio (1403–1443) was appointed ruler of the Duchy of Spoleto by Pope Martin V (1419) and carried on war against Braccio da Montone with varying fortune.
He began his military career under Braccio da Montone, and, returned to his lands, expanded them by hiring mercenaries from the surrounding mountains.