In 1919, the frustrations caused by delays to Irish home rule led members of Sinn Féin, a pro-independence party that had won a majority of the Irish seats at Westminster in the 1918 British general election, to establish an Irish assembly in Dublin, at which Irish independence was declared. The Irish Republican Army simultaneously began a guerrilla war against the British administration. The Anglo-Irish War ended in 1921 with a stalemate and the signing of the Anglo-Irish Treaty, creating the Irish Free State, a Dominion within the British Empire, with effective internal independence but still constitutionally linked with the British Crown. Northern Ireland, consisting of six of the 32 Irish counties which had been established as a devolved region under the 1920 Government of Ireland Act, immediately exercised its option under the treaty to retain its existing status within the United Kingdom.