The Second Opium War, also known as the Arrow War, was a continuation of the conflicts between China and Western powers over issues related to the opium trade. It was triggered by the Chinese capture of the Arrow, a British-registered ship that was suspected of piracy and smuggling. This incident led to increased tension between Britain and China, eventually resulting in a second military confrontation.

One of the significant battles of the Second Opium War was the capture of Canton by British and French forces. The city, a major trading port, fell to the allies in December 1857. Following this, the allied forces advanced towards Beijing, engaging in several skirmishes along the way. The most notable event was the Siege of Beijing, which lasted from June to August 1860 and resulted in the occupation of the city.

The Treaty of Tientsin, signed in 1858, marked a turning point in the war. It expanded upon the terms set in the Treaty of Nanking from the First Opium War and granted further concessions to the Western powers. These included the opening of more Chinese ports for trade, the legalization of the opium trade, and the establishment of diplomatic missions in Beijing.

Another critical outcome of the Second Opium War was the annexation of Hong Kong by the British. With the signing of the Treaty of Tientsin, the island of Hong Kong and the adjacent Kowloon Peninsula were ceded to British control. This annexation facilitated British access to Chinese markets and further strengthened their presence in the region.