"Southern" Xiong-nu living within Han territory. By 31 BC, the Han dynasty abolished universal military conscription that was passed down from the Warring States. In the South, China's territory was roughly doubled as the Chinese conquered much of what is now Southern China, and extended the frontier from the Yangtze to Vietnam. Armies during the Qin and Han dynasties largely inherited their institutions from the earlier Warring States Period, with the major exception that cavalry forces were becoming more and more important, due to the threat of the Xiongnu. Under Emperor Wu of Han, the Chinese launched a series of massive cavalry expeditions against the Xiongnu, defeating them and conquering much of what is now Northern China, Western China, Mongolia, Central Asia, and Korea. After these victories, Chinese armies were tasked with the goal of holding the new territories against incursions and revolts by peoples such as the Qiang, Xianbei and Xiongnu who had come under Chinese rule. The structure of the army also changed in this period. While the Qin had utilized a conscript army, by Eastern Han, the army was made up largely of volunteers and conscription could be avoided by paying a fee. Those who presented the government with supplies, horses, or slaves were also exempted from conscription. Three Kingdoms–Jin The end of the Han dynasty saw a massive agrarian uprising that had to be quelled by local governors, who seized the opportunity to form their own armies. The central army disintegrated and was replaced by a series of local warlords, who fought for power until most of the North was unified by Cao Cao, who laid the foundation for the Wei dynasty, which ruled most of China. However, much of Southern China was ruled by two rival Kingdoms, Shu Han and Wu. As a result, this era is known as the Three Kingdoms. Under the Wei dynasty, the military system changed from the centralized military system of the Han. Unlike the Han, whose forces were concentrated into a central army of volunteer soldiers, Wei's forces depended on the Buqu, a group for whom soldiering was a hereditary profession. These "military households" were given land to farm, but their children could only marry into the families of other "military households". In effect, the military career was inherited; when a soldier or commander died or became unable to fight, a male relative would inherit his position. These hereditary