Explanation:
In the mid-19th century, the British addiction to the dangerous drug known as ‘tea’ created a massive trade imbalance with Qing China. No, really. British taste for tea (and porcelain, and silk) caused an immense amount of silver to flow into China. Like any good merchant, the British simply figured out what good Qing China desired in turn in order to rectify this trade imbalance! Unfortunately, China had little interest in allowing British consumer goods into their markets, and Chinese elites had little taste for British artisan goods.
Luckily, the British stumbled on opium as a desirable trade good! And with their control over India, the Brits set to growing opium in massive amounts and flooding China’s markets with it!
For obvious reasons, when it became apparent that Britain was becoming the trashiest world empire and basing their trade relations on drug dealing, Qing China was not pleased, and restricted imports to destroy the opium trade, the abundance of which had caused opium addiction to become widespread by the drastic drop in price by the increased supply.
Britain did not like this, and decided war was an acceptable solution to keep drug dealing to the Chinese. They won. Twice. And extorted some exorbitant concessions in addition to guarantees for their drug trade.