In January 1919, the Anglo-Irish War began with the first shots being fired at Soloheadbeg. Over the next year, the Royal Irish Constabulary became the target of a Sinn Fein terror campaign. Michael Collins orchestrated this campaign. He felt there would be much to gain by provoking England to war.
By mid-1919, the IRB had infiltrated the leadership of the Volunteers and were directing its pace on the violence. Michael Collins had been made President of the IRB Supreme Council. At the same time, he was Minister for Finance in the Dail government and the commander of the IRA. In June of that year, de Valera left for America and Michael Collins became acting President after Arthur Griffith's arrest in December 1920.
Although Collins and de Valera co-operated, there were differences between them. After the Easter Rising, de Valera had not rejoined the IRB. Cathal Brugha, de Valera's Minister for Defence in the Dail, resented Collins' popularity and his influence over the Volunteers. In an effort to assert control, Brugha had the Volunteers declared the Army of the Irish Republic (IRA).

Britain responded with violence. Special forces were sent over to impose curfews and martial law on the Irish. These forces became known as the Black and Tans after a popular Limerick hunt group, and because of their dark green and khaki uniforms. Another force of veterans from the Great War, called the Auxiliaries, joined them. Thus began a pattern of assassination and reprisal. The IRA employed guerilla tactics, using 'flying columns' to attack British troops. Their knowledge of the countryside made up for their lack of arms. The initial distaste for the killing of RIC men by the IRA gave way to outrage at the savageness of the Crown forces. The reprisals had the effect of identifying the British as the oppressors of the Irish people.
On 21 November 1920 Michael Collins' squad assassinated 14 British officers, effectively destroying the British Secret Service in Ireland. In reprisal, the Black and Tans fired on a crowd watching a football match at Croke Park. Twelve people were killed, including one of the team players. The day became known as Bloody Sunday. News of this and other horrors became known throughout the world.

During this period, Michael, who in the 1918 general election had been elected to Parliament representing South Cork, and Harry Boland, the MP for Roscommon, each vied for the affections of a Longford girl, Catherine Brigid, or more commonly, Kitty Kiernan. From the latter half of 1921 until his death, Michael and Kitty exchanged more than 300 letters. By year's end, Michael had succeeded in winning the fair Kitty and they became engaged. In May of 1921, the IRA set ablaze the Dublin Custom House, but Crown forces arrived in time to capture nearly the entire Dublin IRA Brigade. After this action, the IRA were desperately short of men and weapons, but at the same time, the British were completely demoralised with public opinion increasingly against continued repression. The commander of His Majesty's Crown forces in Ireland advised David Lloyd George to 'go all out or get out.' This began the treaty talks

On 12 July 1921, the day after a truce was signed, de Valera led a delegation to London for exploratory talks with the British Prime Minister. These talks broke down after irreconcilable differences developed over the issue of an Irish Republic--a concession Lloyd George was not about to give. In September of that year, de Valera was elected President of the Irish Republic and he offered to negotiate as representative of a sovereign state. Lloyd George refused. He would allow peace talks only with a view of how Ireland might reconcile their national aspirations within a framework of the community of nations known as the British Empire.

Knowing that neither a Republic nor a united Ireland could be won at such a conference, de Valera refused to attend. Instead, he sent Arthur Griffith and Michael Collins to head the Irish delegation. Neither Griffith nor Collins wanted to go. Michael Collins declared that he was a soldier, not a politician, but the issue went to the Cabinet and was decided by de Valera's casting vote.