In 1918 Aiken was elected chairman of the Armagh Comhairle Ceanntair (District Executive) of Sinn Féin during the nationwide backlash against forced British Army conscription. He was among the organizers who took control of the Sinn Féin movement and turned it into the political arm of the Irish Republicans. Aiken became commander of the 4th Northern Division of the Irish Republican Army in 1921, serving until April 1923. He aligned himself with the anti–Irish Free State Treaty faction, hoping to prevent division and civil war. He ordered the cease-fire and dump arms orders on 24 May 1923 that effectively ended the civil war.
Aiken entered politics in 1923 when he was elected to parliament. He was a founding member of Fianna Fail (Irish Republican Party) and served in a variety of government posts, including minister of finance (1945–1948), minister of agriculture (1957), minister of external affairs (1951–1954), and deputy prime minister (1951–1954, 1957–1969).
Aiken was an early proponent of seating the People's Republic of China (PRC) in the United Nations and worked steadfastly to help smaller nations such as Tibet and Hungary, which fell victim to invasions by the PRC and the Soviet Union, respectively. He also supported self-determination for Algeria, spoke out against apartheid in South Africa, and sought to prevent the proliferation of nuclear weapons. Aiken retired from the Irish parliament in 1973 and died at his home in County Louth on 18 May 1983.
Lydon, James F. The Making of Ireland: A History. London: Routledge, 1998.