The Ming dynasty was the ruling dynasty of China from 1368 to 1644. The Ming was the last dynasty in China ruled by ethnic Han Chinese. Ming rule saw the construction of a vast navy and a standing army of one million troops. There were enormous construction projects, including the restoration of the Grand Canal and the Great Wall (pictured) and the establishment of the Forbidden City in Beijing. Emperor Hongwu's rebuilding of China's agricultural base and strengthening of communication routes through the militarized courier system had the unintended effect of creating a vast agricultural surplus that could be sold at burgeoning markets located along courier routes. By the 16th century, China became involved in a new global trade of goods, plants, animals, and food crops known as the Columbian Exchange. Trade with European powers and the Japanese brought in massive amounts of silver, which then replaced copper and paper banknotes as the common medium of exchange in China. During the last decades of the Ming the flow of silver into China was greatly diminished, thereby undermining the entire Ming economy. The ensuing breakdown of authority and people's livelihoods allowed rebel leaders such as Li Zicheng to challenge Ming authority.