Chinese goods such as silk and tea were very popular among British subjects. However, the Chinese were unwilling to purchase British goods in return. In order to create demand in British trade, the East Indian Company and British government began smuggling and selling highly addictive opium into China.  In 1839, the Chinese government tried to put a stop to the influx of opium flooding their nation and creating addicts of their subjects. They convininced British Superintendent of Chinese Trade, Charles Elliot to give them the stocks of opium waiting in Canton. Once in the Chinese government's possession, they destroyed the opium. This infuriated the British and the fighting began in earnest the following year. China lost the conflict and was required to open more ports for trade, sign over Hong Kong as a British territory, and pay reparations. 

Image Source: https://visualizingcultures.mit.edu/opium_wars_01/ow1_gallery/pages/1841_0792_nemesis_jm_nmm.htm