Charlemagne, learning of the intrigues in Italy, appeared there much sooner than his enemies had anticipated, and easily thwarted their designs; after which he went to Rome and attended the Easter festival (787). Fearing that Charlemagne might discover all his secret plottings, Thassilo sent messengers to the Pope asking him to take steps to bring about a reconciliation between himself and the King. The Pope, however, uncertain whether he was in earnest or simply wished to gain time, not only refused to comply with his request, but sent word to him that if he violated his solemn promises or evaded them in any way the ban would be pronounced against him.
After his return Charlemagne summoned a parliament at Worms and laid Thassilo’s case before it. His refusal to appear only aggravated his guilt. War was declared against him. Three Frankish armies invaded Bavaria, two of them led by the King’s sons, Carl and Pepin, the third by Charlemagne in person. Thassilo was taken by surprise, for he had intended to begin his operations later. Neither the discontented Thuringians, who were in league with him, nor the Slavs and Avars, came to his help. Thassilo’s wife, Luitberger, daughter of Desiderius, had persistently urged her husband to make war against Charlemagne, and when too late she realized the folly of her advice and the danger which threatened Thassilo. The Bavarians themselves were not eager to fight, and indeed expressed more confidence in Charlemagne than in their Duke. Under these discouraging circumstances Thassilo deemed it wisest to betake himself to Charlemagne’s camp and implore his pardon. He was exceedingly penitent and tendered his sceptre to the King, saying that he had forfeited any right to hold it longer. Charlemagne invested Thassilo with the dukedom in fee and took hostages from him, among them his son, Theudo.
Delighted that he had escaped this danger, Thassilo went to Regensburg and Charlemagne returned home. But Thassilo had hardly come under his wife’s influence again when he violated his promise and resumed his hostile machinations. He summoned the leaders of his people to Regensburg, denounced his royal cousin, reviled him, and openly declared he would not respect a compulsory promise even if it cost him ten sons. The foolish Duke did not realize how contemptible he made himself by his conduct in the eyes of all honest men. He renewed negotiations with the Avars and induced them to join him. One division of the barbarians was led by Thassilo through Bavarian territory into the Frankish kingdom, and a second into Italy; but both armies were defeated by Carl, who was sent against them by the King.