In 1931, with the passage of the Statute of Westminster, the Parliament of the United Kingdom relinquished its remaining authority to legislate for the Free State and the other dominions. This had the effect of making the dominions fully sovereign states. The Free State thus became the first internationally recognised independent Irish state.
In the first months of the Free State, the Irish Civil War was waged between the newly established National Army and the anti-Treaty IRA, who refused to recognise the state. The Civil War ended in victory for the government forces, with the anti-Treaty forces dumping their arms in May 1923. The anti-Treaty political party, Sinn Féin, refused to take its seats in the Dáil, leaving the relatively small Labour Party as the only opposition party. In 1926, when Sinn Féin president Éamon de Valera failed to have this policy reversed, he resigned from Sinn Féin and founded Fianna Fáil. Fianna Fáil entered the Dáil following the 1927 general election, and entered government after the Irish general election, 1932, when it became the largest party.
The Treaty allowed for the creation of an independent state to be known as the Irish Free State, with dominion status, within the then British Empire—a status equivalent to Canada. The Parliament of Northern Ireland could, by presenting an address to the king, opt not to be included in the Free State, in which case a Boundary Commission would be established to determine where the boundary between them should lie. Members of the parliament of the Free State would be required to take an oath of allegiance to the king, albeit a modification of the oath taken in other dominions.