Because China had forbidden the opium trade, the British government initiated a military campaign against it in 1839. For years, Britain had illegally smuggled opium into China, and the Chinese government's intention to crack down on the trade was viewed as a direct threat to British interests in the region.
The British government sent a navy fleet to China and bombarded many key Chinese cities, including Canton (now Guangzhou) and Shanghai.
The British also invaded Hong Kong, which later became a British colony. The Opium Wars was a watershed moment in China-Britain relations, ushering in a new age of Western imperialism in China.