Annales regni Francorum
Charlemagne launched a series of military expeditions against the Avars in the last decade of the 8th century which caused the collapse of the Avar Khaganate. The "Royal Frankish Annals" narrates that Avars who "could not stay in their previous dwelling places on account of the attacks of the Slavs" approached Charlemagne in Aachen in 805 and asked to be allowed to settle in the lowlands along the river Rába.