In April 1971, after the arrest and imprisonment of Billy McKee, Cahill became the commander of the Provisional IRA Belfast Brigade. He held this post until the introduction of internment in August of that year. It was during this period that the Provisional IRA campaign got off the ground in the city. Cahill authorised the beginning of the IRA's bombing campaign as well as attacks on British troops and the RUC. He based himself in a house in Andersonstown and toured the city, co-ordinating IRA activity. The day after the British Army mounted Operation Demetrius, designed to arrest the IRA's leaders, Cahill held a press conference in a school in Ballymurphy and stated that the operation had been a failure. He said, "we have lost one brigade officer, one battalion officer and the rest are volunteers, or as they say in the British Army, privates". To avoid the propaganda defeat that his capture would then have entailed, Cahill fled to the Republic of Ireland, temporarily relinquishing his command of the Belfast Brigade.