The Han emperors oversaw leaps in technology, philosophy, religion, and trade. Nonetheless, the Han Empire crumbled away in 221 CE. Why did the Han Empire collapse?
The Han Dynasty in China, one of the world's great classical civilizations, collapsed for both internal and external reasons in 220-221 CE.
Internal Causes:By the 180s the Han court had grown weak, with debauched or disinterested emperors who lived only for amusement. Court eunuchs vied for power with the scholar-officials and army generals, at times even leading to massacres within the palace. In 189 CE, the warlord Dong Zhuo went so far as to assassinate the 13-year-old Emperor Shao, placing Shao's younger brother on the throne instead.
Economically, the Han government had trouble collecting enough tax revenue to fund the court and to support the armies that defended China from external threats. The scholar-officials generally exempted themselves from taxes, and the peasants had a sort of early-warning system by which they could alert one another when the tax collectors came to a particular village. The peasants would scatter to the surrounding countryside, and wait until the tax men had gone. As a result, the central government was chronically short on money.
One reason that the peasants fled at the rumor of tax collectors is that they were trying to survive on smaller and smaller plots of farm land. The population was growing quickly, and each son was supposed to inherit a piece of land when the father died. Thus, farms were quickly being carved into ever-tinier bits, and peasant families had trouble supporting themselves, even if they managed to avoid paying taxes.
Externally, the Han Dynasty faced the same threat that plagued every indigenous Chinese government throughout history - the danger of raids by the nomadic peoples of the steppes. To the north and west, China borders on desert and range-lands that have been controlled by various nomadic peoples over time, including the Uighurs, Kazakhs, Mongols, Jurchens (Manchu), and the Xiongnu.
During prosperous times, the settled agricultural people of China would simply pay tribute to troublesome nomads, or hire them to provide protection from the other tribes. Emperors even offered Chinese princesses as brides to the "barbarian" rulers in order to preserve the peace. The Han government, however, did not have the resources to buy off all of the nomads.
One of the most important factors in the collapse of the Han Dynasty, in fact, may have been the Sino-Xiongnu Wars of 133 BCE to 89 CE. Over more than two centuries, the Chinese and the Xiongnu fought throughout the western regions of China - a critical area that Silk Road trade goods had to cross to reach the Han Chinese cities. In 89 CE, the Han crushed the Xiongnu state, but this victory came at such a high price that it helped to fatally destabilize the Han government.
Interestingly, half of the Xiongnu moved west in the wake of their defeat, absorbing other nomadic groups, and forming a formidable new ethnic group known as the Huns. Thus, the descendants of the Xiongnu would be implicated in the collapse of two other great classical civilizations, as well - the Roman Empire, in 476 CE, and India's Gupta Empire in 550 CE. In each case, the Huns did not actually conquer these empires, but did weaken them militarily and economically, leading to their collapses.
Aftermath:For China, the end of the Han Dynasty marked the beginning of a chaotic era. After a period of civil war and warlordism, the country settled in to the Three Kingdoms period, in which China was divided among the kingdoms of Wei in the north, Shu in the southwest, and Wu in the center and east. China would not reunify for another four hundred years.
Sources on the Collapse of Han China:
Bender, Mark. Introduction to Chinese History, The Ohio State University.
Duiker, William J. & Jackson J. Spielvogel. World History to 1500, Cengage Learning, 2008.