Mongolian Plateau
By the time he died of typhoid fever at the age of 54, Abaoji had captured the entire Balhae state, keeping their ruling family as nobility ruling his new fiefdom. His eastern boundaries were the Yalu River and the Ussuri River. His westward progression had gone far onto the Mongolian Plateau. By the time of his death, he had not acted on his plan to move south.