Desiderius, when he first attacked Rome, was not wrong in thinking that Charles was already occupied in the affairs of his own kingdom. He had that summer commenced the great undertaking of the conquest of Saxony, a task which was to tax his energies for the next twenty years. In the summer of 772 he had entered the land, compelled the Mid-Saxons or Engrians to give him hostages, and cut down in token of triumph the Irminsul, a holy tree reverenced by all the Saxon tribes, which stood in a grove near Paderborn, and was adorned with many rich offerings. On his return to Austrasia, Charles met the ambassadors of Hadrian and Desiderius at Thionville. He did not swerve for a moment from his father’s policy of supporting the Papacy through thick and thin. He sent off ambassadors to bid Desiderius give up all the cities belonging to the Holy See that he was unlawfully occupying, and told him to do justice to St. Peter without delay. [Sidenote: Charles invades Lombardy, 773.] The Lombard king was far too angry at this interference to grant the Frank’s demands. He swore that he would restore nothing. This drew down Charles into Italy. Marching from Geneva he crossed Mont Cenis with one division of his army, while his uncle Bernard with the rest followed the route of the Great St. Bernard. Desiderius on their approach fortified the Alpine gorges by Susa and Ivrea, and stood upon the defensive. But a chosen band of Franks turned his position at Susa by climbing over the hills, and when he saw himself outflanked, the Lombard king abandoned his lines, and fell back on Pavia, exactly as his predecessor Aistulf had done in the war with king Pippin. Charles followed in haste, and laid siege to Pavia, which held out for many months. Meanwhile Adelchis, the son of Desiderius, raised a second Lombard army, and took post in front of Verona. Leaving part of his army to maintain the blockade of Pavia, Charles marched against Adelchis, compelled him to fly, and captured Verona, and afterwards Brescia and Bergamo. The Lombard prince took to the sea, and sought Constantinople, where he endeavoured to obtain help from Constantine Copronymus, then in the midst of his Bulgarian war.