After more than 500 years of British rule, the Anglo-Irish Treaty of 6 December declared 26 counties of Ireland independent, marking the birth of the Irish Free State. The treaty came after a series of rapid political changes which began with the 1918 general election, when a majority of Irish seats in the British parliament was won by the Irish nationalistic Sinn Fein party.

In 1919, the Sinn Fein MPs (Members of Parliament), led by Eamon de Valera, set up an independent Irish parliament, the Dail Eireann, in Dublin.

In the treaty, the British government made a big concession by setting up the Irish Free State. The 26 Roman Catholic-dominated counties in the south comprised a self-governing country with dominion status, while the six mainly Protestant counties of Ulster remained under British rule.