In the initial stages of the conflict in Northern Ireland, the ICO (as it then was) took part along with the IRA in the defence of Catholic areas from Protestant attacks. It was critical of both the and of the people who later created the Provisional IRA. The ICO line was the Two Nations Theory - that the Ulster Protestants were or had the potential to become a nation in their own right, and that Irish Catholics could not determine the whole of the island of Ireland as a country. Their seminal publication on the question was "The Economics of Partition" which ran to many editions. Another important ICO publication was "Connolly and Partition", described by the ICO magazine