Despite his other problems, Charlemagne could not ignore the provocations of the Lombards. At first, hoping to avoid a costly military campaign, he offered Desiderius a substantial financial settlement to leave Rome in peace. But the Lombards rejected this offer.
There was now no alternative. Charlemagne massed an army at Geneva. He put one column under the command of his uncle, Bernard, and instructed him to cross the Alps at the Saint Bernard Pass, which would lead him to Ivrea through the Dora Valley. The king himself led his men towards Turin by a different route and attacked the Lombard army near Susa.
At first Desiderius thought he had outmatched the Carolingian forces, since the strong fortifications which he had erected at Susa stopped Charlemagne’s men. But the Lombard had not counted on the arrival of Bernard, who infiltrated the enemy lines and caught them by complete surprise. There was nothing Desiderius could do but retreat to the city of Pavia and hope to survive the siege which the Franks would inevitably impose.
During that winter, while he remained in Pavia, powerless, Desiderius held hope that the harsh weather would defeat the Frankish troops, and they would be forced to disband. He was also waiting for assistance from his son, Adalgiso, who had been massing an army at Verona.
Both these expectations, however, proved fruitless. The Franks survived the winter intact, and once Adalgiso’s troop had mustered, the Franks defeated them easily, forcing the Lombard prince to flee to Constantinople.
The year was 774. Spring came, and Pavia still held. Finally, during the summer, famine brought the city to its knees, and Desiderius surrendered. On July 10, in Pavia, Charlemagne was crowned rex Francorum et Longobardorum (king of the Franks and the Lombards). Evidently he decided not to make Lombardy an integral part of his kingdom, but instead seized for himself and his heirs the dynastic rights to the Lombard throne. This arrangement permitted him to maintain power over his new territory without having to drain his corps of capable administrators to govern it.