The British used opium to subdue the Chinese masses, Mao just had them worked to near death, tortured and then killed. Where do you think Pol Pot got his ideas from?
The answer lies in the tea.
Tea was imported into Britain in the 17C and was exceedingley expensive. It was paid for, as were all the accoutrements of tea, the pots, cups etc etc, in silver. Tea became synonymous with stately homes and parasols but its purchase endangered the stocks of silver held by Britain.
Meanwhile poppies were grown in India and produced opium, a medicine at that time. It was introduced to China and was used to get back the enormous quantities of silver held by the Chinese.
So the subjugation was the by product of capitalist trade.
The second Opium war, (also called the Arrow War, after a Chinese smuggling vessel the Arrow) came as a result of a complex series of raids and arrests of smugglers in victoria Harbour. The first 'blows' were by two drunken sailors out on the lash and wanting more beer. They walked into a small store, so the story goes, and demanded booze. The owner refused to serve them so they killed him!
Within a short while a British navy ship was firing onto the mainland of kowloon.
There is also an extensive official history which, I'm sure, formed the basis for countless stories of evil foreigners and deeds of derring-do. Part of which involves an island in the Pearl estuary, close to Hong Kong, but within the Chinese borders, called Ling Ding. Ling Ding was the hide out and centre for the opium smuggling trade into Guangzhou.
When I arrived in Hong Kong Ling Ding had a different claim to faim. It was, as I understand, almost entirely populated by prostitutes and brothels and was the only part of the Peoples Republic of Chine that foreigners could visit without a visa. A friend told me that when you get off the boat you just give the guy in uniform a few bucks and go ashore for the night. One of my regrets is that I never visited this place! Just as a tourist, you understand.