On 26 January 1921, McCorley, was involved in the fatal shooting of three
Auxiliary Division officers in their beds in the Railway View hotel in central Belfast. Shortly afterwards, McCorley and another IRA man,
Seamus Woods, organised an Active Service Unit (ASU) within the first battalion of the Belfast Brigade, with the intention of carrying out attacks, with or without the approval of the Brigade leadership. The unit consisted of 32 men. McCorley later wrote, 'I issued a general order that, where reprisal gangs [State forces] were cornered, no prisoners were to be taken'. In March 1921, McCorley personally led the ASU in the killing of three
Black and Tans in Victoria Street in central Belfast. He was responsible for the deaths of two more Auxiliaries in Donegall Place in April. In reprisal for these shootings, members of the RIC assassinated two republican activists, the Duffin brothers in Clonard Gardens in west Belfast. On 10 June 1921 Woods and McCorley units were involved in the killing of an RIC man who was suspected in the revenge killings of the Duffin brothers (see 23 April 1921
Timeline of the Irish War of Independence). Two RIC men and a civilian were also wounded in that attack. Thereafter, there was what historian Robert Lynch has described as a 'savage underground war' between McCorley's ASU and RIC personnel based in Springfield Road barracks and led by an Inspector Ferris. Ferris had also been accused of involvement in the murder of the Lord Mayor of Cork Thomas MacCurtain and had been posted to Lisburn for his safety. During this period of violence, Ferris himself was among the casualties, being shot in the chest and neck, but surviving. McCorley claimed to have been one of the four IRA men who shot Ferris. In addition, McCorley's men bombed and burned a number of businesses including several cinemas and a
Reform Club. In May 1921, however, 13 of McCorley's best men were arrested when surrounded by British troops during an operation in
county Cavan. They were held in Crumlin Road jail and sentenced to death. On 3 June, McCorley organised an attack on Crumlin Road jail in an attempt to rescue the IRA men held there before they were executed. The operation was not a success, however the condemned men were reprieved after a truce was agreed between the IRA and British forces in July 1921. On
Bloody Sunday (10 July 1921) McCorley was a major leader in the defence of nationalist areas from attacks by both the police and loyalists. On that day twenty people were killed. This was despite a nationwide truce that had been agreed between British and Irish leaders, due to come into effect at noon on 11 July. McCorley himself liaised with the RIC to try to ensure the truce held in Belfast but noted later that in Belfast 'the truce lasted six hours only'. At least 100 people were wounded, about 200 houses were destroyed or badly damaged – most of them Catholic homes, leaving 1,000 people homeless. (see:
The Troubles in Ulster (1920–1922)). ==Belfast Brigade & 3rd Northern Division leader==