The start of the Opium War (also known as the First Opium War or the First Anglo-Chinese War) between China and Britain in 1840 (Emperor Daoguang’s 20th year) was widely recognized as marking the beginning of modern Chinese history. It signified an era of development in the economic, political, military, social and cultural areas. Most significantly it marked a switch from an agricultural economy to industrialization.
By the mid-19th century, Britain was a major capitalist power following a period of political and industrial reform. It needed to look overseas for both raw materials and markets for its finished goods and it was prepared to use “gunboat diplomacy” - calling it free trade - to achieve these goals. China at this time was an authoritarian country relying on a self-sufficient economy to support a national policy of seclusion.
Britain initially sought to promote its consumer products in China but failed to gain market access due to heavy Chinese restrictions. So it began selling opium (a highly addictive drug) to China. In 1839, at the order of Emperor Daoguang (道光帝), Lin Zexu (林則徐) arrived in Guangzhou (廣州) to enforce a ban on opium and subsequently seized and destroyed large amounts of opium at Humen (虎門).
The British were determined to force the Qing government to open up China to foreign trade. In June 1840, British vessels arrived in Guangzhou and opened fire; the Opium War officially started. The British fleet then travelled north and entered the Yangtze River near Nanjing(Nanking).
China had a sizeable army under highly decorated officers such as Guan Tianpei (關天培) and Chen Huacheng (陳化成), as well as a civilian resistance force in Sanyuanli near Guangzhou. However, they were not as developed as the British in terms of equipment, training and tactics. Consequently they were defeated by the British equipped with modern weaponry. China had no choice but to negotiate for peace and sign the Treaty of Nanking (南京條約) in August 1842. The terms of this treaty were unequal: China was forced to cede land (Hong Kong Island), pay reparations and open trading ports. China became a semi-colonial state under the rule of foreign powers gradually.