Tassilo, then still a child, began his rule as a
Frankish ward under the tutelage of his uncle, the
Carolingian Mayor of the Palace Pepin the Short (later
king) after Tassilo's father, Duke
Odilo of Bavaria, had died in 748 and Pepin's half-brother
Grifo had tried to seize the duchy for himself. Pepin removed Grifo and installed the young Tassilo as duke, but under Frankish overlordship in 749. In 757, according to the
Royal Frankish Annals, Tassilo became Pepin's
vassal and the lord for his lands at an assembly held at
Compiegne. There, he is reported to have sworn numerous oaths to Pepin and, according to reports that may have been written much later, promised fealty to him and his sons,
Charles and
Carloman. However, the highly legalistic account is quite out of character for the period. K. L. Roper 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. It is reported that Tassilo had gained such a reputation that he was regarded as a kingly ruler when his cousins Charles and Carloman assumed power in the Frankish realm in 768. In the following year, 769, Tassilo issued in
Bolzano the foundation charter of the
Innichen Abbey. He was, however, not able to protect the pope against Lombard expansions, which has been seen as a reason for Rome's lack of support for him during his later conflict with Charlemagne. Still, there is a consensus among historians that Tassilo, despite acting as a kingly sovereign, did not intend to become king himself. In 772, Tassilo sent his son Theodo to Italy to visit the court of his grandfather, Desiderius, and to be baptised by
Pope Adrian I in Rome on May 19. In 773, Tassilo sent an embassy to the pope, but it was blocked by Charles, who was suspicious of the duke's alliances with
Saxons,
Wends, and
Avars. ==Deposition==