The 1842 treaty had not mentioned opium, but the trade continued. Rebellions against the Chinese Government gave foreigners opportunities to shake free of Chinese controls; from these years emerged the Shanghai Municipal Council and the Chinese Maritime Customs, both controlled by the British. Yet merchants chafed at alleged restrictions on trade and diplomats at the lack of access to Beijing.

In 1856, the strains led to the "Second Opium War" (1856-58); when the Chinese proved slow in accepting terms imposed in 1858, there was further fighting, by a Franco-British army. In 1860, additional treaties gave foreigners most of what they sought. Diplomatic missions opened in Beijing; new ports joined the original five; the British added Kowloon to Hong Kong; and China's control over trade and tariffs were further restricted. Further humiliation came with the Boxer incident of 1900, when the siege of the legations led to a huge indemnity; China was now semi-colonised. Sovereignty remained formally intact, but the western powers controlled trade, divided China into spheres of influence, and leased Chinese territory.

British traders dominated the treaty ports and the China trade. After 1895, British manufacturing companies moved into China. In 1898, the New Territories were added to Hong Kong on a 99-year lease, while Weihaiwei in Shandong province was leased as a naval station. British missionaries established schools and colleges. Chinese studies began in British universities, and Chinese art and literature were increasingly appreciated.