Final answer:
After the First Opium War, the British gained control of Hong Kong through the Treaty of Nanjing in 1842, which also opened additional ports to European trade.
Step-by-step explanation:
The Chinese city that the British gained control of after the First Opium War was Hong Kong. The conflict, which occurred from 1839 to 1842, concluded with the signing of the Treaty of Nanjing. Under this treaty, China not only ceded Hong Kong to Great Britain but also opened five ports to European traders, which included Guangzhou, Shanghai, Ningbo, Fuzhou, and Xiamen, and granted Britain a favorable trading status. The First Opium War was a result of tensions over the opium trade, as China sought to curb the illegal importation of opium primarily by the British. When Chinese actions threatened British trade interests, Britain used its superior naval forces to impose its terms. This war was part of the larger pattern of colonial expansion and marked a significant moment of Western dominance in China.