From Britannica:
"The Opium Wars arose from China’s attempts to suppress the opium trade. Foreign traders (primarily British) had been illegally exporting opium mainly from India to China since the 18th century, but that trade grew dramatically from about 1820. The resulting widespread addiction in China was causing serious social and economic disruption there. In spring 1839 the Chinese government confiscated and destroyed more than 20,000 chests of opium—some 1,400 tons of the drug—that were warehoused at Canton (Guangzhou) by British merchants. The antagonism between the two sides increased in July when some drunken British sailors killed a Chinese villager. The British government, which did not wish its subjects to be tried in the Chinese legal system, refused to turn the accused men over to the Chinese courts.
Hostilities broke out later that year when British warships destroyed a Chinese blockade of the Pearl River (Zhu Jiang) estuary at Hong Kong. The British government decided in early 1840 to send an expeditionary force to China, which arrived at Hong Kong in June. The British fleet proceeded up the Pearl River estuary to Canton, and, after months of negotiations there, attacked and occupied the city in May 1841. Subsequent British campaigns over the next year were likewise successful against the inferior Qing forces, despite a determined counterattack by Chinese troops in the spring of 1842. The British held against that offensive, however, and captured Nanjing (Nanking) in late August, which put an end to the fighting.
Peace negotiations proceeded quickly, resulting in the Treaty of Nanjing, signed on August 29. By its provisions, China was required to pay Britain a large indemnity, cede Hong Kong Island to the British, and increase the number of treaty ports where the British could trade and reside from one (Canton) to five. Among the four additional designated ports was Shanghai, and the new access to foreigners there marked the beginning of the city’s transformation into one of China’s major commercial entrepôts. The British Supplementary Treaty of the Bogue (Humen), signed October 8, 1843, gave British citizens extraterritoriality (the right to be tried by British courts) and most-favoured-nation status (Britain was granted any rights in China that might be granted to other foreign countries). Other Western countries quickly demanded and were given similar privileges."
