How did the Anglo-Irish Treaty lead to the Irish Civil War?

The Anglo-Irish Treaty led to the Irish Civil War by creating a deep ideological rift within the Irish nationalist movement over the terms of the agreement. While the treaty established the Irish Free State with dominion status, it required members of the Dáil to take an oath of allegiance to the British monarch, a condition many republicans found unacceptable.123

This disagreement split the Sinn Féin party and the Irish Republican Army (IRA) into pro-treaty and anti-treaty factions. Éamon de Valera, who opposed the treaty, resigned as president of the Dáil after it was ratified by a narrow margin, which provided legitimacy to the anti-treaty opposition.2314

The pro-treaty government, led by Michael Collins, sought to implement the agreement, while the anti-treaty faction—often called "Irregulars"—organized armed resistance. Tensions escalated into open conflict in June 1922 when the Provisional Government attacked anti-treaty forces occupying the Four Courts in Dublin, triggering the Irish Civil War.34