What was the cause and the effect of the second opium war?

The Second Opium War (1856-1860) was primarily caused by disputes over trade, particularly Britain's desire to expand its opium trade in China and the Chinese government's attempts to suppress it. The conflict resulted in a series of military confrontations, ultimately leading to the Treaty of Tientsin and the Convention of Peking, which forced China to concede to numerous foreign demands. The war significantly expanded foreign influence in China, legalized the opium trade, and marked a further decline of Qing authority, contributing to internal unrest and weakening the imperial state.