Complex sentence: The Poliane and the area of Kiev had before the advent of the Rus'formed the western frontier of Khazaria, so despite their commercial contacts, the Rus' and Khazars were rivals.
Split into simpler sentences: The Poliane and the area of Kiev itself had before the advent of the Rus'formed Khazaria's western frontier. So, despite their commercial contacts, the Rus' and Khazars were rivals.