What caused the partition of Ireland in 1920?

The partition of Ireland in 1920 was primarily driven by the intense opposition of Ulster unionists to the prospect of being governed by a nationalist-dominated Irish state.12

Following the rise of the revolutionary party Sinn Féin and the outbreak of the Irish War of Independence, the British government sought a political solution to the Irish question. Responding largely to the demands of unionists—who feared the loss of their British identity, culture, and political privilege—the British Parliament enacted the Government of Ireland Act in 1920.132

This legislation divided the island into two self-governing regions:
* Northern Ireland: Comprising six counties (Antrim, Down, Armagh, Londonderry, Fermanagh, and Tyrone) where unionists maintained a permanent majority.12
* Southern Ireland: Comprising the remaining 26 counties.12

While Sinn Féin rejected the act, the partition was solidified by the subsequent Anglo-Irish Treaty, which granted Northern Ireland the option to opt out of the newly formed Irish Free State, a choice it ultimately exercised.13