The primary cause of the Opium War (1839-1842) between Britain and China was the British trade of opium from India into China, which had devastating effects on Chinese society. The Chinese government, alarmed by the increasing addiction rates and social problems tied to opium, enforced strict bans on the drug. In response, the British sought to protect their lucrative opium trade, leading to heightened tensions. The conflict escalated when China confiscated and destroyed opium stockpiles. Ultimately, the war led to the Treaty of Nanking, which imposed harsh terms on China, including ceding Hong Kong to Britain and opening several ports to foreign trade, significantly altering the dynamics of Sino-British relations.