What caused civil war to break out in Ireland in the 1920s according to a country divided?

The civil war in Ireland during the 1920s was primarily caused by divisions over the Anglo-Irish Treaty, which established the Irish Free State as a self-governing dominion within the British Empire. Many nationalists, particularly those aligned with the anti-Treaty faction led by Éamon de Valera, opposed the treaty for its compromise on full independence and continued allegiance to the British Crown. This disagreement led to a split within the nationalist movement and ultimately resulted in armed conflict between pro-Treaty forces, who supported the new government, and anti-Treaty forces, who sought full sovereignty. The war reflected deep-seated political, ideological, and cultural divides within Irish society.