The Oath of Allegiance clause in the Anglo-Irish Treaty of 1921, which obliged all elected representatives to swear allegiance to the British monarch, was the major cause of the Civil War. The anti-Treatyites were defeated but Sinn Fein emerged with forty-four Dail seats in the 1923 general election. Gradually, Sinn Fein leader Eamon de Valera became convinced that the party's policy of abstaining from Dail Eireann because of the Oath was untenable. In 1926 he proposed 'that Sinn Fein elected deputies enter the Dail or Six-County Assembly if the Oath of Allegiance were removed.'

His motion was defeated and he resigned as President of Sinn Fein, afterwards founding a new political party, Fianna Fail. In the general election of June 1927, the vast majority of Republican voters voted for Fianna Fail and forty-four of the party's deputies were elected. These deputies declared that the Oath was just 'an empty formula' and took their Dail seats. After a long political battle, Fianna Fail, backed by the Labour Part, pushed a Bill through the Dail and Senate to abolish the Oath in May 1933. Although initially the British government regarded this as a breach of the 1921 Treaty, it finally accepted the abolition of the Oath in 1937 when de Valera's new constitution was endorsed by referendum.