一口通商
The restrictions imposed by the Qianlong Emperor that established the Canton System were highly lucrative for Guangzhou's Cohong—the merchant Howqua became one of the world's wealthiest individuals—and normalized Guangzhou's tax base and inflow of foreign silver. Under the canton system, the Qianlong Emperor restricted trade to only licensed Chinese merchants, while the British government on their part issued a monopoly charter for trade only to the British East India Company. This arrangement was not challenged until the 19th century when the idea of free trade was popularised in the West. The Canton system did not completely affect Chinese trade with the rest of the world as Chinese merchants, with their large three-masted ocean junks, were heavily involved in global trade. By sailing to and from Siam, Indonesia and Philippines, they were major facilitators of the global trading system; the era was even described by Carl Trocki as a "Chinese century" of global commerce. Chinese merchants could also trade freely and legally with Westerners (Spanish and Portuguese) in Xiamen and Macao, or with any country when trade was conducted through ports outside China such as Manila and Batavia. By restricting imports mostly to bullion, however, it created strong pressure on the British—for whom tea had become the national drink over the course of the 17th century—to find any means possible to adjust the balance of trade. This turned out to be opium grown on planations in India, which became so lucrative and important that the viceroy Lin Zexu's vigorous enforcement of existing laws against the smuggling of opium prompted the First Opium War and the beginning of the unequal treaties that restricted Qing sovereignty in the 19th century. The 1842 Treaty of Nanking opened the ports of Xiamen ("Amoy"), Fuzhou ("Fuchow"), Ningbo ("Ningpo"), and Shanghai, but legal trade continued to be limited to specified ports to the end of the dynasty.