After a short civil war, a new dynasty, called Han (206 BC to 220 AD), emerged with its capital at Chang'an. The new empire retained much of the Qin administrative structure but retreated a bit from centralized rule by establishing vassal principalities for the sake of political convenience. The Han rulers modified some of the harsher aspects of the previous dynasty. Technological advances also marked this period with two inventions, paper and porcelain. The Han dynasty was notable for its military prowess. The empire expanded westward to modern Xinjiang Province. After 200 years, Han rule was interrupted briefly (in AD 9 to 24 by Wang Mang, a reformer), then restored for another 200 years. The Han rulers, however, were unable to adjust to what centralization had wrought: a growing population, increasing wealth and resultant financial difficulties and rivalries, and ever-more complex political institutions. Riddled with corruption characteristic of the dynastic cycle, by AD 220, the Han Empire collapsed.