 The Bulgarians' main rivals in the area were the Avars to the west and the Khazars to the east.The Khazars were a serious threat; they marched westwards after they crushed the resistance of Kubrat's eldest son Bayan and waged a war against Asparukh, who perished in battle in 700.To protect their northern borders, the Bulgarians built several enormous ditches that ran the whole length of the border from the Timok river to the Black Sea.