There were two wars that consisted of the Opium Wars. They are mostly fought between China and Great Britain in the 1800s.
Before the First Opium War, Great Britain was selling opium to China, which is a highly addictive drug. They were making a lot of money off of China, as many people started to get addicted. Over time, China started to get fed up with this. They were sick of seeing their people get more and more addicted to opium and wanted to put a stop to it. They tried to ask Great Britain to stop selling the Chinese people opium, but they continued to do so. This ended up breaking out in a war.
During the First Opium War, Great Britain easily moved into China with its navy. They were much more equipped and had newer machines, which China did not. They easily destroyed the Chinese Naval units and continued to move inwards. They ended up opening the opium trade in China, which China had previously closed, and now Great Britain had full control over Chinas trade. They took control over cities, people, and held an obvious lead.
China knew that they were not going to come back from this or win, they needed to make a negotiation. The negotiation is called the Treaty of Nanking, this made China give Great Britain a very large sum of money, access to Hong Kong, and opened up trade with the countries.