<human>:  When did the British start trading opium with China?
<bot>: The British started trading opium with China in the 1840s. The British East India Company had a monopoly on the opium trade in China. They bought opium from the Chinese farmers, then sold it to the Chinese government. The British East India Company made a lot of money off the opium trade. They used the money to build ships and pay for the troops that protected their opium trade routes.