The partition of Ireland was the process by which the Government of the United Kingdom of Great Britain and Ireland (UK) divided Ireland into two self-governing polities: Northern Ireland and Southern Ireland. It was enacted on 3 May 1921 under the Government of Ireland Act 1920. The Act intended both territories to remain within the United Kingdom and contained provisions for their eventual reunification. The smaller Northern Ireland was duly created with a devolved government and remained part of the UK. The larger Southern Ireland was not recognised by most of its citizens, who instead recognised the self-declared 32-county Irish Republic. On 6 December 1922 Ireland was partitioned. At that time the territory of Southern Ireland left the UK and became the Irish Free State, now known as the Republic of Ireland.

Ireland was part of the United Kingdom from 1801 to 1922. For almost all of this period, the island was governed by the UK Parliament in London through its Dublin Castle administration in Ireland. Ireland underwent considerable difficulties in the 19th century, especially the Great Famine of the 1840s which started a population decline that continued for almost a century. The late 19th and early 20th centuries saw a vigorous campaign for Irish Home Rule. While legislation enabling Irish Home Rule was eventually passed, militant and armed opposition from Irish unionists, particularly in Ulster, opposed it. Proclamation was shelved for the duration following the outbreak of World War I. By 1918, however, moderate Irish nationalism had been eclipsed by militant republican separatism. In 1919, war broke out between republican separatists and British Government forces. Subsequent negotiations between Sinn Féin, the major Irish party, and the UK government led to the signing of the Anglo-Irish Treaty, which resulted in five-sixths of the island seceding from the United Kingdom, becoming the Irish Free State, with only the six northeastern counties remaining within the United Kingdom.