How did Charlemagne’s conquests change the eastern frontier of the Frankish kingdom?

Charlemagne’s military campaigns significantly expanded and transformed the eastern frontier of the Frankish kingdom through a combination of conquest, annexation, and the establishment of new administrative zones.

The most demanding of these efforts was the long-term war against the Saxons, which spanned over 30 years (772–804) and resulted in the annexation of a large territory between the Rhine and Elbe rivers. Additionally, the forced annexation of Bavaria in 787–788 brought the Franks into direct contact with the Avars, an Asiatic nomadic empire. Subsequent campaigns against the Avars led to the disintegration of their power and allowed Charlemagne to claim territories in Carinthia and Pannonia south of the Danube.1

To secure these ever-lengthening borders, Charlemagne utilized a mix of military force and diplomacy to establish relatively stable relations with various Slavic tribes stretching from the Baltic Sea to the Balkans. These efforts effectively shifted the Frankish frontier significantly eastward, integrating new regions into the realm and opening them to Christian missionary work.1