[citation needed] His sister, Nano, married Phelim Magennis a Newry stockbroker, and was the last female republican prisoner to be released in Northern Ireland at the end of the Civil War.

In 1917, making an outward display of defiance, Aiken raised the republican Irish tricolour, opposite Camlough Barracks in Armagh, a move designed as deliberate provocation.

[6] In March 1918 he was arrested by the RIC for illegal drilling — an act of open defiance that provoked a sentence of imprisonment for one month.

By 1919 Aiken's clandestine activities mainly consisted of arms raids on dumps of the Ulster Volunteers who had imported weapons to resist Home Rule in 1913–14.

As well as UVF dumps, Aiken and the Newry Brigade also raided prominent local unionist barracks at Dromilly, Ballyedmond Castle and Loughall Manor.

During the First Dáil Election he headed a team from South Armagh who went to Carlingford to assist the Sinn Fein candidacy of J.J. Kelly in Louth where they were attacked by members of the local Ancient Order of Hibernians.

[9] Operating from the south Armagh/north Louth area, Aiken's unit was one of the most effective IRA in Ulster during the Irish War of Independence.

[10][11] In May 1920, he led 200 IRA men in an assault on the RIC barracks in Newtownhamilton, attacking the building and then burning it with paraffin spayed from a potato sprayer; however, the garrison did not surrender.

[12] Aiken himself led a squad which blasted a hole in the wall of the barracks with gelignite and entered through it, exchanging shots with the policemen inside.

In reprisal, the newly formed Ulster Special Constabulary burned Aiken's home and those of ten of his relatives in the Camlough area.

After the IRA split over the Anglo-Irish Treaty in 1922, Aiken ultimately became aligned with the anti-Treaty side in spite of personal efforts to prevent division and civil war and by remaining neutral at first.

After fighting broke out between pro- and anti-Treaty units in Dublin on 28 June 1922, he wrote to Richard Mulcahy on 6 July 1922 calling for a truce, the election of a new reunited IRA army council and the removal of the Oath of Allegiance from the Free State constitution.

Subsequently, Aiken travelled to Limerick meet with anti-Treaty IRA leader Liam Lynch, and urged him to consider a truce in return for the removal of references to the British monarch from the constitution.

Aiken was attempting throughout to avoid Civil War from breaking out and urging that all military remain together to push north to re-take the Six-Counties of Northern Ireland , territory which made up his homeland and counties of his own Army Division.

[28] Despite his neutrality and pleas for a negotiated end to the Civil War, Aiken was arrested by pro-Treaty troops on 16 July 1922, under Dan Hogan, on the orders of Mulcahy and Collins, and imprisoned at Dundalk Gaol along with about 200 to 300 of his men.

Always ambivalent about the war against the Free State, Aiken soon issued a letter ordering a suspension of offensive operations from 30 April.Department of the Chief of Staff, General Headquarters, Dublin, April 27th, 1923.

[34]Aiken had remained close to Éamon de Valera, who had long wanted to end the Civil War, and together the two came up with a compromise that would save the anti-Treaty side from a formal surrender.

Aiken was an innovator, an amateur inventor who took out patents throughout his life for a turf stove, a beehive, an air shelter, an electric cooker and a spring heel for a shoe among others and also powered the family home in Sandyford through use of a wind turbine.

The King arranged a special deal between both men, whereby McNeill would retire from his post a few weeks earlier than planned, with the resignation coinciding with the dates de Valera had suggested for the dismissal.

[citation needed] At the outbreak of Second World War, a period known in Ireland as The Emergency, Aiken was appointed as Minister for the Co-ordination of Defensive Measures by de Valera with Oscar Traynor assuming the role of Minister for Defence, in de Valera's first major cabinet reshuffle as the country was put on a wartime footing to defend Irish neutrality.

[citation needed] He gained notoriety in Dublin media circles for overseeing censorship: his clashes with R. M. Smyllie, editor of The Irish Times, ensured this censorious attitude was resented by many.[who?]

[41][page needed] However, it was the former government minister, Joseph Connolly, and fellow Ulster founder member of Fianna Fáil, who was appointed to the office of censor.

In diplomatic negotiations Aiken told him that a united Ireland, if it was conceded, would still stay neutral to safeguard its security and that further talks were 'a sheer waste of time'.

[46] In March 1941, Aiken was sent to America to secure US supplies, both military and economic, that DeValera claimed Britain was withholding owing to Irish neutrality.

Roosevelt urged Aiken and Ireland to join the war on the allied side asking if it was true that he had said that 'Ireland had nothing to fear from a German victory'.

[48] Aiken was Minister for Finance for three years following the war and was involved in economic post–war development in the industrial, agricultural, educational and other spheres.

Despite a great deal of opposition, both at home and abroad, he stubbornly asserted the right of smaller UN member countries to discuss the representation of communist China at the General Assembly.

[52] After his retirement, the outgoing President of Ireland Éamon de Valera, sought to convince Aiken—one of his closest friends—to run for Fianna Fáil in the 1973 presidential election.

[31] The couple first met in County Wicklow in the house of Aiken's friend, James Ryan, who helped encourage the relationship.

Having been denied the right, through an exclusion order, to attend his family home the former IRA Commander's dying wish was respected by local Fianna Fáil organisation, the Dundalk & District Old IRA and by the 27 Infantry Battalion of the barracks that now bears his name who followed his funeral cortege through Dundalk to the border out of respect to their fallen comrade.