overall market. Commerce was relatively frequent, increasing after the Han Dynasty with the development of the silk road The Wu Hu uprising crippled the economy, which did not recover until the Tang, under which it transformed into the mercantile economy of the Song and Ming Dynasties. Qin dynasty (221–206 BCE) In 221 BCE, the State of Qin conquered the remaining states in China, becoming what scholars consider the first unified Chinese state. It quickly expanded, extending the Southern frontier from the Yangtze to modern Vietnam, and the northern frontier to modern Mongolia. While the early Xia, Shang, and Zhou Dynasties had nominal authority over all of China, the feudal system gave most regions a large degree of autonomy. Under the Qin, however, a centralised state was established, and the entire empire had uniform standards and currency to facilitate trade. In addition, the Qin government undertook many public works projects, like the Great Wall of China. The Qin initiated what is considered the "first Chinese empire", which lasted until the Wu Hu uprising. The Qin emperor unified standards of writing, weight measurement, and wheel length, while abolishing the old currencies, which varied between states. He also issued a uniform code of laws throughout the empire, which made trade easier. Defensive walls between states were demolished because of their disruptive influence upon trade. The Qin Empire did not return to the old feudal system, but set up a system of 36 commandries, each governing a number of counties. Other Qin policies including heavy taxation of salt and iron manufactures, and forced migration of many Chinese towards new territories in the south and west. The Qin government undertook many public works projects, the workers often being conscripted by the state in order to repay a tax debt. The most famous of these projects is the Great Wall of China, built to defend the state against Xiongnu incursions. Other Qin projects include the Lingqu Canal, which linked subsidiaries of the Yangtze and Pearl rivers and made possible the Qin's southern conquests, as well as an extensive road system estimated at . However, Qin's legalist laws and the heavy burden of its taxes and corvee were not easily accepted by the rest of the empire. Unlike the other states, Qin specifically enacted laws to exile merchants and expropriate their wealth, as well as imposing monopolies on salt, iron, forests and other natural resources. Scholars note