First World War

Dalton joined the British Army in 1915 for the duration of the Great War. His decision was not that unusual among Irish Volunteers, as over 20,000 of the National Volunteers joined the British New Army on the urgings of Nationalist leader John Redmond. Dalton's father, however, disagreed with his son's decision. Emmet Dalton initially joined the 7th Battalion of the Royal Dublin Fusiliers (RDF) as a temporary 2nd Lieutenant.^[1][4][5] By 1916, he was attached to the 9th Battalion, RDF, 16th (Irish) Division under Major-General William Hickie, which contained many Irish nationalist recruits.

During the Battle of the Somme in September 1916, Dalton was involved in bloody fighting during the Battle of Ginchy, in which over 4,000 Irishmen were killed or wounded.^[4] Among the casualties was Tom Kettle, a former nationalist Member of Parliament and personal friend of Dalton's father and of Emmet. Dalton was awarded the Military Cross for his conduct in the battle.^[6][7] Afterwards he was transferred to the 6th Battalion, Leinster Regiment, and sent to Salonika then Palestine, where he commanded a company and then supervised a sniper school in El Arish.^[4] In 1918 Dalton was re-deployed again to France, and in July promoted to captain, serving as an instructor.^[8][4]

Irish War of Independence

On demobilisation in April 1919, Dalton returned to Ireland.^[4] There, finding that his younger brother Charlie had joined the IRA, Dalton himself followed suit. Dalton later commented on the apparent contradiction of fighting both with and against the British Army by saying that he had fought for Ireland with the British and fought for Ireland against them.^[8]

He became close to Michael Collins and rose swiftly to become IRA Director of Intelligence – and was involved in the Squad, the Dublin-based assassination unit. On 14 May 1921, Dalton led an operation with Paddy Daly that Dalton and Collins had devised. It was designed to rescue Gen. Sean McEoin from Mountjoy Prison using a hijacked British armoured car and two of Dalton's old British Army uniforms.^[4]

Irish Civil War

Dalton followed Collins in accepting the Anglo-Irish Treaty in 1922 and was one of the first officers – a Major General – in the new National Army established by the Irish Provisional Government of the Irish Free State. The Treaty was opposed by much of the IRA and the Irish Civil War between pro and anti-treaty factions eventually resulted.

Dalton was in command of troops assaulting the Four Courts in the Battle of Dublin which marked the start of the war in June 1922. At Collins' instigation Dalton, as military liaison officer with the British during the truce, took control of the two 18-pounder guns from the British that were trained on the buildings. He became commander of the Free State Army under Mulcahy's direction. He was behind the Irish Free State offensive of July–August 1922 that dislodged the anti-treaty fighters from the towns of Munster. Dalton proposed seaborne landings to take the anti-treaty positions from the rear, and he commanded one such naval landing that took Cork city in early August.^[2] In spite of firm loyalty to the National Army, he was critical of the Free State's failure to follow up its victory, allowing the anti-treaty IRA to regroup resuming the guerrilla warfare started in 1919.^[9]

On 22 August 1922, he accompanied Michael Collins in convoy, touring rural west Cork. The convoy was ambushed near Béal na Bláth and Collins was killed in the firefight. Dalton had advised him to drive on, but Collins, who was not an experienced combat veteran, insisted on stopping to fight.^[10][7]

Dalton was married shortly afterwards (on 9 October 1922) to Alice Shannon in Cork's Imperial Hotel.^[4] By December 1922 he had resigned his command in the Army. He did not agree with the execution of republican prisoners that marked the latter stages of the Civil War. After briefly working as clerk of the Irish Senate, he left this job to work in the movie industry.