The first Opium war left an indelible scar on China. The mainland lost Hong Kong and was forced to open up trade to foreigners.
In the 18th century, foreign trade with China was limited to Canton, modern-day Guangzhou. Foreigners were confined to towns outside of Canton, known as the '13 Factories', or Hongs (not really factories). British trade was run by the East India Trading Company; Chinese trade was dominated by the Hongs.
Here is a timeline of what happened:
1820 - Import of opium begins in earnest
China is willing to provide Britain with tea and other luxury goods, but is unwilling to accept anything but silver as payment. The British have to import silver from Europe or Mexico. They run into a trade deficit and seek ways to counter-trade. They find a solution in an Indian narcotic: opium. In the next few years, the amount of opium imported to China increases dramatically. Tensions arise because, in China, opium can only be used as a medicine. It has been banned as a recreational drug for more than 100 years.
April 1839 - Lin Zexu is sent to Canton and 20,000 chests of opium are burnt
