Why was Eamon de Valera imprisoned by the Irish Free State government?

Eamon de Valera was imprisoned by the Irish Free State government because he supported the republican resistance during the Irish Civil War (1922–1923).1

Following the ratification of the Anglo-Irish Treaty by the Dáil Éireann, de Valera resigned as president in protest, primarily due to the treaty's requirement of an oath of allegiance to the British crown. His opposition to the treaty and his subsequent support for the anti-treaty forces led to his imprisonment by the ministry of William Thomas Cosgrave. De Valera remained in custody until his release in 1924.12