Two conflicts between Britain and China (1839–1842 and 1856–1860) fought over British insistence on the right to sell opium in China and China's attempts to prohibit the trade. The Opium Wars inaugurated China's 'century of humiliation,' opened Chinese markets and territory to foreign powers through unequal treaties, and remain defining events in Chinese national memory and foreign policy.
Britain went to war with China to protect the right to sell drugs. That sentence sounds like a caricature but it is the plain historical fact. The British East India Company grew opium in India and sold it in China to balance its accounts — Chinese tea and silk were in enormous demand in Britain, but the British had little that Chinese buyers wanted in return. Opium solved the trade imbalance. When the Chinese government destroyed twenty thousand chests of British opium in 1839, Britain sent warships. The resulting treaties gave Britain Hong Kong, opened five ports to foreign trade, and established the principle of extraterritoriality — that British subjects in China were immune from Chinese law. It was the first of many such defeats.