The Provisional and Official IRA decided to intensify their campaigns after British Prime Minister Edward Heath decided to impose direct rule from Westminster.
But following an iniative to bring the Provisional IRA and the British government to talks, the republicans declared a brief "bilateral truce" in June 1972.
Republican leaders, including a young Gerry Adams (released from internment) and Martin McGuinness, were secretly taken to London for talks with the Northern Ireland Secretary William Whitelaw. The talks failed.
Two weeks later on Friday 21 July, the Provos detonated at least 22 bombs in Belfast city centre. The day became known as Bloody Friday.
The 65-minute attack killed 11 people and seriously injured 130 as the IRA detonated car bombs, mines and other devices. Some of the victims's bodies had been blown to pieces, leading to some of the most shocking scenes of the troubles as their remains were collected in black plastic bags.
The horrific and terrifying nature of the attack led many nationalists to conclude that even if they had previously understood - and perhaps accepted - the emergence of a defensive organisation, the Provisional IRA had passed far beyond what could ever be deemed as a justifiable action in the name of the Catholic community.
The paramilitaries, many concluded, had given up any right to be part of the political solution - they were now clearly part of the problem.
Sectarian division had reached such a point that loyalist paramilitaries were also establishing "no-go areas" in North Belfast and some hardline unionist politicians were actively talking about a unilateral declaration of independence.
The Northern Ireland Secretary William Whitelaw came under increasing pressure to take action and on 31 July he ordered 20,000 soldiers to dismantle IRA barricades in the no-go areas of Derry and Belfast.
The troops met little resistance. That evening, car bombs killed eight people in the small town of Claudy in County Derry. No-one claimed responsibility.
1972 became the bloodiest year of The Troubles. Some 470 people were killed that year, the overwhelming majority of them civilians.
There were more than 10,600 shootings and almost 1,400 explosions. Approximately 500 further devices were defused. In some 36,000 house searches, the army seized approximately 1,200 firearms, 183,000 rounds of ammunition and 19,000kg of explosives.