Complex sentence: The Avars had already been mixing with the more numerous Slavs for generations, and they later came under the rule of external polities such as the Franks, Bulgaria, and Great Moravia.
Split into simpler sentences: The Avars had already been mixing with the more numerous Slavs for generations. In turn, they came under the rule of external polities--that of the Franks, Bulgaria, and Great Moravia.