By a mutual exchange of good offices, in less than three years Pepin crossed the Alps at the head of a numerous army, overthrew the Lombards, and recovered the Italian territory which they had wrested from the Eastern empire. Justice would indeed have demanded that it should be returned to the Emperor to whom it belonged; or he might have retained it for himself. But he did neither. Mindful of his obligation to the holy See, he replied, that he had not gone to battle for the sake of any man, but for the sake of St. Peter alone, and to obtain the forgiveness of his sins. He then transferred the sovereignty over the provinces in question to the bishop of Rome. This was the foundation of the whole temporal dominion of the popes.
Astolph, king of the Lombards, having sworn to Pepin that he would restore to St. Peter the towns which he had seized, the French troops were withdrawn. But the magnificent "donation," so far as the pope was concerned, was only on paper. He had not been put into actual possession of the ceded territories, neither had he the means of putting himself in possession of the royal gift. No sooner, therefore, had the Frankish king recrossed the Alps than Astolph refused to fulfill his engagements. He collected his scattered divisions, and resumed his attacks upon the scattered territories of the church. He wasted the country up to the very walls of Rome, and laid siege to the city. The pope, incensed as much at the evasive conduct of Pepin as at the perfidy of the Lombards, sent messages to his Frankish protectors in all haste by sea, for every way by land was closed by the enemy. His first letter reminded King Pepin, that he hazarded eternal condemnation if he did not complete the donation which he had vowed to St. Peter. A second letter followed, more pathetic, more persuasive. Still the Franks were tardy. And finally the pope wrote a third, as from St. Peter himself. The daring and assumption of this letter is so awful, that we give it entire as a specimen of the means used by the pope to terrify the barbarians into the protection of the Holy See and the advancement of her dominions. He considered all means justifiable for such high purposes. Thus it reads:
"I, Peter the apostle, protest, admonish, and conjure you, the most Christian kings, Pepin, Charles and Carloman, with all the hierarchy, bishops, abbots, priests and all monks; all judges, dukes, counts, and the whole people of the Franks. The mother of God likewise adjures you, and admonishes and commands you, she as well as the thrones and dominions and all the host of heaven, to save the beloved city of Rome from the detested Lombards. If ye hearken, I, Peter the apostle, promise you my protection in this life and in the next, will prepare for you the most glorious mansions in heaven, and will bestow on you the everlasting joys of paradise. Make common cause with my people of Rome, and I will grant whatever ye may pray for. I conjure you not to yield up this city to be lacerated and tormented by the Lombards, lest your own souls be lacerated and tormented in hell with the devil and his pestilential angels. Of all nations under heaven the Franks are highest in the esteem of St. Peter; to me you owe all your victories. Obey, and obey speedily; and, by my suffrage, our Lord Jesus Christ will give you in this life length of days, security, victory; in the life to come, will multiply His blessings upon you, among His saints and angels."