The Sino-British Joint Declaration was a treaty signed between the United Kingdom (UK) and the People's Republic of China (PRC) in 1984 that set the conditions in which the entirety of colonial Hong Kong would be transferred to Chinese control and how the territory shall be governed after 1 July 1997.
Hong Kong first became a colony of the British Empire in 1842 after it had defeated Qing China in the First Opium War, acquiring Hong Kong Island. Its territory was expanded on two occasions; first in 1860 with the addition of Kowloon Peninsula and Stonecutters Island during the Second Opium War and the Taiping Rebellion, and again in 1898 when the UK obtained a 99-year lease for the New Territories shortly after the First Sino-Japanese War. From 1941 to 1945, the colony was occupied by the Japanese Empire but returned to British control after World War II. The date of the handover in 1997 marked the end of these leases.
The PRC government declared in the treaty its basic policies for governing Hong Kong after the transfer. A special administrative region (SAR) would be established in the territory that would be self-governing with a high degree of autonomy, except in foreign affairs and defence. Hong Kong would maintain its existing governing and economic systems separate from that of mainland China under the principle of "one country, two systems". This blueprint would be elaborated on in the Hong Kong Basic Law (the post-handover regional constitution) and the central government's policies for the territory were to remain unchanged for a period of at least 50 years after 1997.
Since 2014, the PRC government has stated that it considers the treaty to be spent with no further legal effect, while the UK government maintains that the document still remains binding in operation. Following the PRC's government imposition of a national security legislation on Hong Kong in 2020 and a 2021 National People's Congress decision to approve a rework of local election laws that reduces the number of regional legislature seats elected by the public, the UK has declared the PRC as being in a "state of ongoing non-compliance" with the Joint Declaration.
Background
Phases of colonial expansion
Hong Kong became a British colony in 1842 after the Qing dynasty's defeat in the First Opium War. The territory initially consisted only of Hong Kong Island and was expanded to include Kowloon Peninsula and Stonecutters Island in 1860 after another Qing loss in the Second Opium War.
After this addition, the British government resisted calls by colonial officials and merchants for further expansion into China, deciding to cease territorial acquisitions in the area. However, as Germany, Japan, and Russia coerced China into granting concessions in the late 1890s, the United Kingdom considered another expansion of Hong Kong to bolster the colony's defence against attack from these other great powers. When France obtained a lease for Guangzhouwan, only 210 miles away from Hong Kong, the United Kingdom entered into negotiations with the Qing to acquire the New Territories as a compensatory concession. The British negotiator considered the 99-year lease term to be sufficiently suggestive that the ceded area would be a permanent cession, which was granted rent-free.
Potential lease renegotiations
Hong Kong Governor Frederick Lugard first proposed renegotiating the lease as a formal cession in 1909 in return for restoring Weihaiwei to China. Although the Colonial and Foreign Offices considered this, the plan was never brought to the Chinese government before Weihaiwei was transferred to the Republic of China in 1930. Alternate plans that involved offering China a substantial loan and settling land disputes on the Burmese border were suggested, but these ideas similarly did not progress past discussion. The start of the Second World War and ensuing Japanese occupation of Hong Kong stifled further debate on obtaining a cession. The British government initially prepared for the possibility of having to relinquish Hong Kong after the war, but moved towards retaining the colony over the course of the conflict. Hong Kong returned to British control in 1945.
In the immediate post-war period, the Nationalist government continued this dialogue with the British about the future of Hong Kong, which included discussions of a full retrocession and proposals of turning the colony into an international city. However, communist victory in the Chinese Civil War made a transfer of Hong Kong to the Nationalists an increasingly unlikely scenario, and the status quo was maintained.