Jin reunified the remainder of China in 280. The centralised state weakened after losing the tax revenue of those under the protection of large landowners. Nevertheless, the economy recovered slightly under the policies of Cao Cao, who nationalised abandoned land and ordered peasants to encourage peasants to work on them, as well as re-instituting the hated iron monopoly, which would continue under Jin and the Southern Dynasties. Later, the War of the Eight Princes, which, coupled with the a Jin policy of moving the barbarian Wu Hu into China to alleviate labour shortages caused the eventual fall of China. Wu Hu (304–420) In 304, there was a massive uprising of barbarian clans that were under Chinese control. Following several years of civil war between the various princes of the Jin dynasty, the Xiongnu, led by Liu Yuan, revolted and declared their independence from China. Several other groups under Chinese control subsequently revolted, including the Jie, Qiang, Di, and Xianbei. These groups became collectively known as the Wu Hu, committing genocide against Han Chinese in the Yellow River valley. The collapse of the Jin Empire quickly followed these revolts. The Wu Hu invasion caused large-scale depopulation and the market economy that developed during the Qin, Han, Wei and Jin to decline rapidly, allowing the feudal manorial economy returned to prominence. Large manorial estates were self-sufficient and isolated from the wider market. These estates were self-contained economies that practised agriculture, herding, forestry, fishing, and the production of handicraft goods. Exchange was no longer carried out through money, but barter. Following the economic revival after the unification of the Sui, the manorial economy again declined. A brief economic recovery occurred in the North under Fú Jiān, the Di ruler of Former Qin, who reunified North China. Fu Jian repaired irrigation projects constructed under Han and Jin, and set up hotels and stations for traders every 20 kilometres. Under Fu Jian's rule, trade and agriculture were significantly revived, and many nations again sent tribute to Fu Jian's court. This recovery ended when Fu Jian was defeated at the battle of Fei by Jin forces, causing his empire to collapse into a series of small states. This battle is considered one of the most important in Chinese history because it preserved Chinese civilization from the danger of destruction. Meanwhile, the legitimate Jin dynasty had fled to the south, then an undeveloped periphery of the Chinese