On December 19, 1984, Prime Minister Margaret Thatcher of the United Kingdom signed a “joint declaration” with her counterpart from the People’s Republic of China, Zhai Siyang, formalizing the return of the British colony of Hong Kong to China on July 1, 1997.

The United Kingdom first claimed the island of Hong Kong as a colony in 1842 after imposing a treaty on China’s Qing dynasty following a conflict known as the First Opium War. After a subsequent conflict–the Second Opium War–the British forced China to cede adjacent territory on the Kowloon peninsula in 1860. In 1898, the British demanded even more land for their expanding colony, which led to a 99-year lease agreement for what became known as the “New Territories.” Although this lease technically applied only to the New Territories, the 1984 joint declaration provided for the return of the entire colony, which only made practical sense since by the 1980s most of Hong Kong’s population resided in that area.