Deasy opposed the
Anglo-Irish Treaty. In the months that followed, he tried to persuade Colins to renegotiate aspects of the treaty, especially to remove an oath to the British king from the constitution of the new
Irish Free State. When fighting broke out in Dublin in June 1922, between pro and anti-Treaty forces, Deasy sided with the
Anti-Treaty IRA in the ensuing
Irish Civil War. However, he was reluctant to fight his former comrades and voiced the opinion that the fighting should have ended with the Free State seizure of the
Four Courts. In late July, Deasy commanded 1,500 anti-treaty fighters who held a line around
Kilmallock south of
Limerick city against about 2,000 Free State troops under
Eoin O'Duffy. Deasy's men were the most experienced IRA fighters of the 1919-21 war and held their position until 8 August, when they were outflanked by
seaborne landings on the southern coast. Deasy's men then dispersed. He went on the run in the south-east of the country. In August 1922, Deasy was in command of a band of republican guerrillas in
West Cork, when they heard that Collins was in the area. Deasy had his men prepare an ambush for Collins' convoy at
Béal na Bláth, should it return by the same route it had taken earlier. Deasy and most of his men did not take part in the ambush as they had retired to a nearby pub, assuming that they had missed Collins. However, Collins arrived as the last of Deasy's men were clearing the mine and barricade that had been erected on the road at Béal na Bláth. Collins was killed in the ensuing firefight. Deasy later wrote in his memoirs that he profoundly regretted the death of his former commander. ==Capture and surrender==