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Benevento, Bavaria, and Pepin's revolt

Charlemagne travelled to Italy in 786, arriving by Christmas. Aiming to extend his influence further into southern Italy, he marched into the Duchy of Benevento. Duke Arechis fled to a fortified position at Salerno before offering Charlemagne his fealty. Charlemagne accepted his submission and hostages, who included Arechis's son Grimoald. In Italy, Charlemagne also met with envoys from Constantinople. Empress Irene had called the 787 Second Council of Nicaea, but did not inform Charlemagne or invite any Frankish bishops. Charlemagne, probably in reaction to the perceived slight of the exclusion, broke the betrothal of his daughter Rotrude and Constantine VI.

After Charlemagne left Italy, Arechis sent envoys to Irene to offer an alliance; he suggested that she send a Byzantine army with Adalgis, the exiled son of Desiderus, to remove the Franks from power in Lombardy. Before his plans could be finalised, Aldechis and his elder son Romuald died of illness within weeks of each other. Charlemagne sent Grimoald back to Benevento to serve as duke and return it to Frankish suzerainty. The Byzantine army invaded, but were repulsed by the Frankish and Lombard forces.

As affairs were being settled in Italy, Charlemagne turned his attention to Bavaria. Bavaria was ruled by Duke Tassilo, Charlemagne's first cousin, who had been installed by Pepin the Short in 748. Tassilo's sons were also grandsons of Desiderius, and a potential threat to Charlemagne's rule in Lombardy. The neighbouring rulers had a growing rivalry throughout their reigns, but had sworn oaths of peace to each other in 781. In 784, Rotpert (Charlemagne's viceroy in Italy) accused Tassilo of conspiring with Widukind in Saxony and unsuccessfully attacked the Bavarian city of Bolzano. Charlemagne gathered his forces to prepare for an invasion of Bavaria in 787. Dividing the army, the Franks launched a three-pronged attack. Quickly realizing his poor position, Tassilo agreed to surrender and recognise Charlemagne as his overlord. The following year, Tassilo was accused of plotting with the Avars to attack Charlemagne. He was deposed and sent to a monastery, and Charlemagne absorbed Bavaria into his kingdom. Charlemagne spent the next few years based in Regensburg, largely focused on consolidating his rule of Bavaria and warring against the Avars. Successful campaigns against them were launched from Bavaria and Italy in 788, and Charlemagne led campaigns in 791 and 792.

Charlemagne gave Charles the Younger rule of Maine in Neustria in 789, leaving Pepin the Hunchback his only son without lands. His relationship with Himiltrude was now apparently seen as illegitimate at his court, and Pepin was sidelined from the succession. In 792, as his father and brothers were gathered in Regensburg, Pepin conspired with Bavarian nobles to assassinate them and install himself as king. The plot was discovered and revealed to Charlemagne before it could proceed; Pepin was sent to a monastery, and many of his co-conspirators were executed.

The early 790s saw a marked focus on ecclesiastical affairs by Charlemagne. He summoned a council in Regensburg in 792 to address the theological controversy over the adoptionism doctrine in the Spanish church and formulate a response to the Second Council of Nicea. The council condemned adoptionism as heresy and led to the production of the Libri Carolini, a detailed argument against Nicea's canons. In 794, Charlemagne called another council in Frankfurt. The council confirmed Regensburg's positions on adoptionism and Nicea, recognised the deposition of Tassilo, set grain prices, reformed Frankish coinage, forbade abbesses from blessing men, and endorsed prayer in vernacular languages. Soon after the council, Fastrada fell ill and died; Charlemagne married the Alamannian noblewoman Luitgard shortly afterwards.