The pact between Pope Stephen and Pepin, in which the Franks were obliged to defend papal interests, was not to Carloman’s liking, and he left his monastery for the court. Pepin arrested him and imprisoned him at Vienne, where he died shortly afterwards. Other nobles opposed the arrangement, and in order to win backing for his decision, the king called a general assembly of the Franks at Quierzy-sur-Oise. The Neustrians, in particular, stood against any alliance with Rome.The men of the Frankish court, however, had not expected the sophisticated arguments of Pope Stephen himself, who had arrived to lend his support to Pepin, and, in the end, the pact won the approval of the assembly.
It was not long after this assembly that Stephen called upon Pepin to live up to his word and send an army against the Lombard king, Aistulf. At first, Pepin tried to bribe Aistulf for the lands he had promised the pope. Aistulf refused, and Pepin led his men across the Alps into Italy.
Aistulf’s cavalry met the Franks on the banks of the Dora. Pepin repeated the strategy his father used at Poitiers, and his infantry repulsed repeated charges of the Lombards and pushed them back to the city of Pavia, where they laid siege.
Aistulf surrendered, and Pepin returned north. As soon as winter had closed the Alpine passes; however, Aistulf raised a fresh army and marched directly against Rome. Rome held until the spring, when Pepin again fought Aistulf and this time defeated them. The church received its promised territory, and a new political entity called the Papal States took control of most of the countryside between Ravenna and Rome.