One year to the day after the signing of the Anglo-Irish Treaty Ireland is partitioned. Northern Ireland and the Irish Free State come into existence.

The 1921 Anglo-Irish Treaty, commonly known in Ireland as The Treaty and officially the Articles of Agreement for a Treaty Between Great Britain and Ireland, was an agreement between the government of the United Kingdom of Great Britain and Ireland and the government of the Irish Republic that concluded the Irish War of Independence. It provided for the establishment of the Irish Free State within a year as a self-governing dominion within the "community of nations known as the British Empire", a status "the same as that of the Dominion of Canada". It also provided Northern Ireland, which had been created by the Government of Ireland Act 1920, an option to opt out of the Irish Free State, which was exercised by the Parliament of Northern Ireland.

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 territory was created with a devolved government and remained part of the UK. Although the larger Southern Ireland was also created, its administration was not recognised by most of its citizens, who instead recognised the self-declared 32-county Irish Republic.

Northern Ireland is a part of the United Kingdom in the north-east of the island of Ireland. It has been variously described as a country, province or region. Northern Ireland shares an open border to the south and west with the Republic of Ireland. At the 2021 census, its population was 1,903,175, making up around 3% of the UK's population and 27% of the population on the island of Ireland. The Northern Ireland Assembly, established by the Northern Ireland Act 1998, holds responsibility for a range of devolved policy matters, while other areas are reserved for the UK Government. The government of Northern Ireland cooperates with the government of Ireland in several areas under the terms of the Good Friday Agreement. The Republic of Ireland also has a consultative role on non-devolved governmental matters through the British–Irish Governmental Conference (BIIG).