 In 430, under the able general Dao Yanzhi, Liu Song recovered the four cities of Luoyang, Hulao, Huatai and Qiao'ao south of the Yellow River.However, the emperor's unwillingness to advance past this line caused the destruction of the empire's ally, Xia, by the Wei.The emperor was to repeat this mistake as several northern states such as Northern Yan who had offered to ally with Liu Song against Wei were declined, eventually leading to Wei's unification of the North in 439.