For most of China's history, its view on global trade has been somewhat lopsided. On the one hand, China always had a lot of cool stuff to export that the rest of the world really wanted. Silk, porcelain, and tea just to name a few. Even as early as Roman times, China was the only source for many of these luxury goods, and thus benefited from the flow of silver and gold into the country. However, while China was pretty cool with the whole exporting thing, it was much less down with idea of imports. After all, imports meant that gold and silver would flow out of the country, and China was all about that bling. To limit imports, China restricted overseas trade to only a few cities in southern China and required all transactions to be done through Chinese merchants. In this way, the Chinese rich could have access to cool things from Europe, but not to a level that would reverse the flow of gold.
This system lasted for several hundred years until the British entered the picture. The British had a real hard on for laissez faire economics, and the world’s most powerful navy to back them up. The very thought of China restricting trade put them into a tizzy, but not nearly as much as the fact that the widespread popularity of Chinese tea in England was emptying the country's coffers of silver and gold. This was a bit of a concern at the time, given that the various British colonies were all losing money. None more so than India, where the cash crop of cotton was no longer able to compete with increased production from the United States, probably because the U.S. was using slaves. However, being clever bastards, the British found a way to solve both their problems in one fell swoop.
That fell swoop was opium. Around 1780, British merchants shipped the first cargo of Indian opium to China, which proved quite popular amongst the Chinese given that opium (which is what morphine and heroine are derived from) apparently is pretty awesome to smoke (you know, if you just ignore all of the negative health effects). It was the perfect answer. Farmers in India could grow opium poppies instead of cotton, which would then be sold to China, thus slowing the flow of British silver and gold into the country. Now opium smoking wasn't a new thing in China, but the stuff the British brought from India could be had for a much cheaper price, which as one can imagine, resulted in more people smoking opium. This trend was further exacerbated when the Americans, not to be outdone, began bringing in lower quality opium from Turkey, driving down prices and making the drug even more affordable for the Chinese masses.
At first, the Chinese government was pretty cool with the whole thing, mostly because the British selling opium led to them having more money to buy tea. However, they soon became much less okay with it when an estimated 25 percent of their male population became addicted to it (fun fact, opium addicts aren't exactly the most productive people in the world), and more importantly, because it was reversing the flow of gold and silver from into China to out of China. In 1838, China declared opium imports illegal and started seizing and destroying cargoes. Britain responded by sending its navy to China to blow the shit out of everything within cannon range of the coast for the next three years in what became known as the First Opium War. Given the technological differences between Britain and China at the time, the war was a lot like a grown man beating up a kid in middle school. The Chinese were forced to sign a treaty in 1842, which along with relegalizing the opium trade, also opened many new ports for trade and gave Hong Kong to the British.
Now obviously this solved nothing for China, so they spent the next fourteen years attempting to modernize their military so that it wouldn't be so easy to have their asses handed to them. It was basically a Rocky training montage for a whole country. It was a long fourteen years of getting bullied by every western country imaginable, but finally in 1856, China was ready to do some flexing of its own. What followed was known as the Second Opium War, which was basically another four years of China getting its ass beat. In the end, China, fully humiliated, did the only thing it could; legalize the production of opium within its own borders. While this did nothing to stop China's huge opium addiction problem, it did at least ensure that people were smoking a home grown product, ending the need to import it from India.
Image: https://en.wikipedia.org/wiki/First_Opium_War#/media/File:Destroying_Chinese_war_junks,_by_E._Duncan_(1843).jpg