Due to Emperor Daowu's cruelty, he was killed by his son Tuoba Shao, but crown prince Tuoba Si managed to defeat Tuoba Shao and took the throne as Emperor Mingyuan. Though he managed to conquer Liu Song's province of Henan, he died soon afterward. Emperor Mingyuan's son Tuoba Tao took the throne as Emperor Taiwu. Due to Emperor Taiwu's energetic efforts, Northern Wei's strength greatly increased, allowing them to repeatedly attack Liu Song. After dealing with the Rouran threat to his northern flank, he engaged in a war to unite northern China. With the fall of the Northern Liang in 439, Emperor Taiwu united northern China, ending the Sixteen Kingdoms period and beginning the Northern and Southern dynasties period with their southern rivals, the Liu Song.