A fervent nationalist, de Valera was one of the leaders of the Easter Rising in 1916 and was sentenced to death by the British, but was released a year later. He served as leader of Sinn Fein (1917–26) and President of the self-declared Irish government (1919–22), but as an opponent of the Anglo-Irish Treaty headed the militant republicans in the ensuing civil war. In 1926 he founded the Fianna Fáil Party, which he led in the Dáil. In 1932 de Valera became President of the Irish Free State, and was largely responsible for the new constitution of 1937, which created the sovereign state of Eire. He served as Taoiseach (Prime Minister) (1937–48; 1951–54; 1957–59) and President (1959–73).