As the 10th century progressed, the Khazars were no longer able to command tribute from the Volga Bulgars, and their relationship with the Byzantines deteriorated, as Byzantium increasingly allied with the Pechenegs against them. The Pechenegs were thus secure to raid the lands of the Khazars from their base between the Volga and Don rivers, allowing them to expand to the west. Rus' relations with the Pechenegs were complex, as the groups alternately formed alliances with and against one another. The Pechenegs were nomads roaming the steppe raising livestock which they traded with the Rus' for agricultural goods and other products. The lucrative Rus' trade with the Byzantine Empire had to pass through Pecheneg-controlled territory, so the need for generally peaceful relations was essential. Nevertheless, while the Primary Chronicle reports the Pechenegs entering Rus' territory in 915 and then making peace, they were waging war with one another again in 920. Pechenegs are reported assisting the Rus' in later campaigns against the Byzantines, yet allied with the Byzantines against the Rus' at other times.