Sino-British Joint Declaration

The Sino-British Joint Declaration was a treaty signed in 1984 between the governments of China and the United Kingdom which set the conditions in which Hong Kong was transferred to Chinese control and for the governance of the territory after 1 July 1997. Hong Kong had been a colony of the British Empire since 1842 and its territory was expanded on two occasions; first in 1860 with the addition of Kowloon Peninsula and Stonecutters Island, as both were ceded in perpetuity, and again in 1898 when Britain obtained a 99-year lease for the New Territories.

Source: Wikipedia — Sino-British Joint Declaration (CC BY-SA 4.0)