After his brother Ratchis became king, Aistulf succeeded him in Friuli. He succeeded him later as king when Ratchis abdicated, and entered into a monastery. While his brother had been more tolerant with the Roman element of the Italian population, Aistulf followed a more aggressive policy of expansion and raids against the Papal States and the Eastern Roman exarchate of Ravenna. In 750, he conquered Comacchio and Ferrara. In 751, he captured Ravenna itself and even threatened Rome, claiming a capitation tax for any inhabitant of the Roman duchy. He also conquered the Istria region from Eastern Roman occupation in the summer of the same year.
Pope Stephen II, thoroughly irritated and alarmed, and despairing of aid from the Roman Emperor, turned to the Carolingian mayors of the palace of Austrasia, the effective rulers of the Frankish kingdom. In 753, after Aistulf had refused any attempt of diplomatic negotiation, Stephen decided to visit Pepin the Short, who had been proclaimed king of the Franks in 751 with the consent of Pope Zachary. Aistulf replied with an alliance with Pepin's brother, Carloman, who was living at Montecassino since 747. However, the latter's attempt of organizing a resistance in France was thwarted, and Carloman was again forced into a monastery. In the Spring of 755 Pepin crossed the Alps, and defeated Aistulf in the Val di Susa, in northern Piedmont. A first treaty was thus signed (June 755) at Pavia, with Aistulf forced to return the conquered territories to the Byzantines.
However, once the Frank army was back behind the Alps, Aistulf besieged again Rome in January-March 756. After the Alpine passes became again free, in April, Pepin moved again into Italy, again defeating the Lombard king the Susa valley and besieging him at Pavia. Aistulf surrendered, paying one third of the Lombard treasury to the Franks, and assigning to the pope the lands that he had torn from the ducatus Romanus and the exarchate (Emilia-Romagna and the Pentapolis).
Aistulf died in a hunting accident in 756. He was succeeded by Desiderius as king of the Lombards and by Alboin as duke of Spoleto. He had given Friuli to his brother-in-law Anselm, abbot of Nonantula, whose sister Gisaltruda he had married, when he succeeded to the kingship in 749.