Not only did British traders profit greatly from opium trade. British government benefited as well. Therefore, destruction of opium stocks in Guangzhou by Lin Zexu also meant huge losses to British government. They dispatched 48 warships equiped with 50 cannon to the coast of Gunagdong, launching First Opium War against China in June 1840.

Under the command of Lin Zexu, the army and people of Guangzhou were well prepared for the war. British fleet then turned up north along the coast and captured Dinghai in Zhejiang province. They reached Tianjin in August, when the panic-stricken Emperor Daoguang sent Minister Qi Shan to negotiate with British. emperor promised to dismiss Lin Zexu as long as British troops went back to Guangdong. When they did so, Li Zexu was removed from his office.

In January 1841, British occupied Hong Kong Island. Chinese government of Qing Dynasty had to declare war against Britain and send Minister Yi Shan to Guangzhou to direct the battle. In February, British troops bombarded the fort in Humen, where Chinese defenders under General Guan Tianpei fought against the enemy without reinforcements. When the fort was lost, General Guan and his brave soldiers continued to fight, using swords, but were overwhelmed and slaughtered. In May, the chicken-hearted Yi Shan surrendered Guangzhou to the British, who met the local people strong resistance, notably from Sanyuanli in suburb of Guangzhou, and incurred serious losses.

The war escalated with British attack of several places in Zhejiang province, including Dinghai, Zhenhai and Ningbo. Chinese forces, some led by the heroic General Ge Yunfei, could not stop British troops because of outdated weapons, inexperienced army officers, and corrupted government officials. In June 1842, British troops captured the Fort Wusong in Shanghai, where they met General Chen Huacheng, who was in his 70s, deeply wounded and bleeding all over, fighting to his last breath with the commanding flag still in his hand. From Fort Wusong the British went upstream along Yangtze River and advanced on Nanjing (Nanking) in August. Chinese government of Qing Dynasty was forced to suurender by signing the Treaty of Nanking. Under the terms of the treaty, China was to pay 21 million silver dollars, to cede Hong Kong Island to Britain, and to open the cities of Guangzhou, Shanghai, Xiamen, Fuzhou and Ningbo as trading ports. The Treaty of Nanking was the first unequal treaty signed by Qing Dynasty. It marked the start of China decline into a semi-colonial and semi-feudal state.