Excerpts from Books and Wikipedia
"Charlemagne . . . had succeeded to the throne in 768 jointly with his brother Carloman. At the time there was antagonism between not only the two ruling brothers, but between the king of the Lombards, Desiderius, and the papacy. In 772, Pope Hadrian I expelled all the Lombard officials from the papal curia. In response, Desiderius invaded papal territory . . . Hadrian called Charlemagne for assistance. Charles had produced an alliance with the Lombards by marrying one of Desiderius' daughters, Desiderata; within a year, however, he had changed his mind about the marriage and alliance, and divorced his wife, sending her back to her father. This was taken as an insult by the Lombards. . . . The relationship between Frank and Lombard now broke down completely and the pope took full advantage. . . . Charlemagne ascertained the truth of Desiderius' aggressions and the threat he posed to his own Frankish realm and marched his troops towards Italy in the early summer of 773." [Wikipedia] "The defeat and consequent destruction of the Lombard monarchy rid Rome of its most persistent threat to papal security, laying the groundwork for the Holy Roman Empire."