The creation of a unified empire by Charlemagne required quite a number of victims, one of whom was Tassilo III, the last duke of the Agilolfing dynasty reigning in Bavaria for two centuries. The history of his fall may awake the legal historians’ interest because the Frank monarch dethroned him not by means of a bloody military defeat but by a legal trial (now called show trial1) in 788. Before the trial Charlemagne isolated Tassilo both in foreign and home affairs by means of carefully measured diplomatic steps. Finally, putting him under his jurisdiction in 787, he made him his vassal. The main charges brought against Tassilo were infidelitas, i.e., unfaithfulness to the liege lord and harisliz, i.e., desertion – though the latter was claimed to had been carried out a quarter of a century before the legal trial. The given work aims to enlighten the legal background of this rather opaque case by contouring the historical context. First we consider Tassilo’s reign and the historical background of the trial, then we investigat the Franko-Bavarian conflict and the iuramenta fidelitatis of Tassilo. In the end, after highlighting the question of infidelitas and of harisliz we analyse the show trial itself.