Pepin removed Grifo and installed the young Tassilo as duke, but under Frankish overlordship. Later, in 757, according to the Royal Frankish Annals, Tassilo became Pepin’s vassal for his lands at an assembly held at Compiègne. However, this highly legalistic account is quite out of character for the period.
K. L. Pearson has suggested that it probably represents a reworking of the original document by the annalist to emphasise Charlemagne’s overlordship over Tassilo during the period of hostilities between the two rulers.
Around 760 Tassilo married Liutperga, daughter of the Lombard king Desiderius continuing a tradition of Lombardo-Bavarian connections. He made several journeys to Italy to visit his father in law and to establish political relations with the pope.
That year he founded Gars Abbey on the Inn River in southern Bavaria. He was however not able to protect the pope against Lombard expansions which has been seen as a reason for Rome"s lack of supporting Tassilo in his later conflict with Charlemagne.
Still, there is consensus among historians that Tassilo despite his acting as a kingly sovereign did not intend to become king himself.
Pearson suggests that he left out of a feeling of obligation to the Aquitanians in light of an earlier alliance made between Tassilo’s father and the Aquitanian duke during his conflict with Pepin in 743. Whatever the motivations behind Tassilo"s abandoning the campaign, the Royal Frankish Annals for that year are particularly scathing of him, saying that he "brushed aside his oaths and all his promises and sneaked away on a wicked pretext". Working on the premise, argued by Pearson, that these annals may have been revised to emphasise Tassilo as a vassal suggests that this was the beginning of a campaign to depict Tassilo as an oath-breaker and as one unprepared to carry out the main function of his office, namely, to fight, making him unfit for rule.
This incident was the linchpin in Charlemagne and Pope Hadrian I’s argument that Tassilo was not an independent prince, but a rebellious vassal deserving punishment.
This punishment was carried out, after much political maneuvering during a diet in the Imperial Palace Ingelheim, in 788, when Tassilo was finally deposed and entered a monastery. In 794, Tassilo was made once more, at the synod of Frankfurt, to renounce his and his family"s claims to Bavaria.