Across the country, the IRA split into pro-Treaty or anti-Treaty forces. Many followed Collins, accepting that the Treaty gave the country the freedom to win freedom. Richard Mulcahy, the Minister of Defence, transformed these loyal troops into the Free State Army, while the anti-Treaty forces became known as the Irregulars.

Collins made every effort to avoid a civil war. He drafted a new constitution which he hoped would be acceptable to the Republicans. The rebels had been Collins' comrades-in-arms and he desperately wanted to avoid such a tragedy, but his efforts failed. In a move to dislodge Republican troops who had taken over the building, on June 28th, Collins ordered the shelling of the Four Courts.

In a controversial move, he armed both pro- and anti-Treaty IRA members in the North to defend the Catholic population, but by resorting to violence against the Treaty terms in the North, he legitimised armed resistance in the South. On 6 July 1922, the Provisional Government appointed a Council of War and Collins became Commander-in-Chief of the national Army.
Opponents of the Treaty rallied to their cause. Fighting broke out in Dublin and Cathal Brugha was killed. The ten-month civil war had begun. The first phase was bloody and brief. By August, the better-equipped government forces had driven the Irregulars out of the main cities and towns, but the Republicans controlled much of the country area to the south and west.
On 12 August 1922, Arthur Griffith died of a massive hemorrhage. He had never recovered from the strain of the Treaty negotiations.