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Tassilo III was duke of Bavaria from 748 to 787, the last of the house of the Agilolfings.

Tassilo, then still an infant, began his rule as a Frankish ward under the tutelage of the Merovingian Mayor of the Palace Pepin the Short (later king) after Tassilo's father, Duke Odilo of Bavaria, had died in 747 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.

Later, in 757, according to the Royal Frankish Annals, Tassilo became Pepin’s vassal for his lands at an assembly held at Compiègne. There he is reported to have sworn numerous oaths to Pepin and promised fealty to him and his sons, Charles and Carloman. However, this highly legalistic account is quite out of character for the period; K. L. Pearson has suggested^[1] 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.

From the Frankish point of view, in 763 Tassilo defaulted on his military obligations to Pepin, leaving the Frankish campaign in Aquitaine on grounds of ill health. 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, in 787, 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.

He married Liutperga, the daughter of Desiderius of Lombardy.