The Opium war was fought in China between the British and the Chinese. It started when the British were importing opium into China, and the government tried to stop it. At this, the British attacked the Chinese, and war ensued.
In the beginning, opium was not an illegal import in China. The British East India Company was importing opium into China because… they didn't have anything else to import. The Chinese weren't interested in silver (they had plenty of that), spices, or anything else the British had, so they started importing opium.
However, the drug started making people addicted, and the British bought more and more opium. The Chinese government looked at its steadily growing population of opium addicts and then decided that they had to do something about it, so they banned the sale of opium.
This didn't stop the British (and now American) importers, who kept selling the drug, and eventually China had to threaten that they will stop giving the British china (the ceramics) and spices.
Then, the Chinese viceroy Lin Zexu discovered 1000 tons of illegal opium and proceeded to dump it in the ocean, Boston Tea Party – style. Britain first demanded reparations, and after none showed up, declared war on the Chinese.
The war really started when British warships arrived to reinforce the merchant ships, and attacked the Chinese, who fought back but were defeated because their enemies had better technology.
In the end, the British forced the Chinese to open up to opium trading and demanded reparations for the destruction of the opium and the cost of the war to the British. Basically, the Chinese got robbed. Oh yeah, and they also had to give the British Hong Kong Island because… they felt like owning it? DOES THIS SEEM LIKE A FAIR DEAL???