The revision of the Southern Irish Constitution in 1937 combined a claim to Ulster and the primacy of the Catholic Church. With what appeared to be the implicit approval of sections of the Southern Irish political establishment, the Irish Republic Army (IRA) continued its struggle to regain Ulster.
Following Eamon de Valera's clamp down on the IRA in Southern Ireland, the first campaign to free Ulster came in mainland Britain. Between January and July 1939 the IRA caused 145 explosions in England, mostly of nuisance value. The campaign rapidly ran into trouble due to effective action by the British security services and a crackdown on the IRA in Southern Ireland. The IRA also tried to make use of links with Nazi Germany but attempts to infiltrate German agents with IRA support failed. The security forces had dealt the IRA a number of serious blows and by 1940 it the IRA had been marginalised and was practically non-existent.
Despite efforts to steal arms from armouries in mainland Britain, the focus shifted back to terrorist operations in Ulster with the 1956 to 1962 cross-border campaign. This campaign was aimed at one of the focal points of republican attention - partition - but was a total failure. Internment for suspected IRA activists was introduced north and south of the border, severely restricting terrorist activities.
Long-standing discrimination against Catholics in Northern Ireland provided the next opportunity for republican terrorism. Since the early 1960s pressure had been mounting to end the provision of services and employment on the basis of religious beliefs, as Catholics suffered employment, housing and political discrimination. The American civil rights campaign and the increasing confidence of middle-class, aspirational Catholics provided the spur for the movement.
First, in 1964 came the Campaign for Social Justice, but it was not interested in traditional nationalist and republican partition issues. By 1967 the civil rights movement had developed an umbrella organisation, the Northern Ireland Civil Rights Association, which also concentrated on internal issues rather than pressing for change on accepted nationalist and republican grievances dating back to the Irish civil war and partition.
The moderate unionist reaction was to accommodate some of the wishes of the civil rights campaigners, while hard-line loyalist wanted no concessions granted. Following protest marches that turned violent in August 1968, reforms were announced in November. But in January 1969 loyalist extremists attacked a peace march and relations between the Catholic and Protestant communities continued to deteriorate. The Protestant marching season sparked massive sectarian rioting, in Londonderry and then Belfast. No-go areas were set up in Catholic housing estates and both Catholic and Protestant districts were cleared of people of the other religion by violent mobs.
The IRA was unsure what to do and in January 1970 it split between the 'officials' who rapidly lapsed into irrelevance, and the 'provisionals' who sought to bomb the British and Protestants out of Ireland. The violence provided an opportunity for the Provisional IRA to establish itself as the defender of Ulster Catholics from Protestant mobs.