The primary cause of the Opium Wars between Britain and China in the 19th century was the British trade of opium in exchange for Chinese tea and other goods. As demand for opium rose in China, the negative societal impacts became significant, prompting the Chinese government to take action against the drug trade. In response, Britain sought to protect its lucrative trade interests, leading to military conflict. The resulting treaties, especially the Treaty of Nanking in 1842, favored Britain and established "unequal treaties" that expanded foreign influence in China, leading to significant social and economic upheaval within the country.