By the time of the Song dynasty, northern China was ruled by the Khitan-led Liao dynasty, the Jurchen-led Jin dynasty, and the Tangut-led Western Xia dynasty. After being threatened by these northern states and realizing the possible effects of a war to the country and people, the Song rulers invented a false concept of kinship with the Jurchens in an attempt to improve relations. The Mongols divided Chinese citizens into two types during the Yuan dynasty: those of the south, and those of the north. When the Ming dynasty expelled the Mongols and reunited China under Han rule, the concept of "tianxia" returned largely as it was during the Han dynasty.