14/15 August 1969: The British Army was deployed on the streets of Derry and Belfast by the British Prime Minister Harold Wilson to stave off the collapse of the Northern State. This was to try and stem the serious rioting in both cities and in other urban centres across the North and to stop the collapse of the State that could come about if the situation continued to spiral out of control.

In response to the growing Crises the Irish Prime Minister An Taoiseach Jack Lynch had gone on the airwaves the previous day to announce the setting up of Field Hospitals near the Border and Refugee Camps further south to deal with the expected influx of people fleeing their homes. This gesture however only seriously angered and worried moderate Unionists and inflamed the more hard line and paranoid Loyalists - while doing nothing of real material benefit to help the beleaguered Nationalists at that time.

While the situation calmed down in Derry as the RUC were withdrawn from the Bogside& the British Army took up positions there the situation slid out of control in Belfast.  There was also serious rioting in Armagh, Newry & Omagh and other areas throughout the North. In Armagh a man was shot dead by the RUC. Five people were killed in overnight rioting in Belfast, one of them a nine year old boy. As the sectarian clashes worsened houses and business premises were set alight and hundreds were damaged or destroyed. Bombay street was totally destroyed and the Catholic residents had to flee for their lives. 

It soon became clear that the discipline of a considerable number of the regular RUC and more particularly the B-Specials had collapsed. Numerous individuals from these organisations went on the rampage and became indistinguishable from the Loyalist mobs on the loose that night.

While the situation in the Six Counties had became much more dangerous over the Summer the multiple deaths in open sectarian clashes was a huge shock to the people of Ireland. For the first time in decades people had been killed in almost open warfare between the Orange and the Green. It was a watershed in Modern Irish Politics.

The troops arrived in the city in jeeps and large lorries which were covered over with protective steel mesh. About 100 troops jumped out and took up positions along Victoria Quay with rifles and sub-machine guns at the ready. They then moved in to Waterloo Place and are now in the act of sealing off William Street and Waterloo Street which lead into the Bogside.
While smoke still bellows over the Bogside, the troops are armed with gas and are wearing gas masks as they mingle with members of the ‘B’ Specials.
A RUC spokesman stated that in the future, the ‘B’ Specials will carry rifles and guns. The job of the troops is to protect the commercial premises in Waterloo Place and the nearby police station at Victoria Barracks.
Bernadette Devlin voices her opinions on the arrival of the troops from behind the barricades in the Bogside, “There can be no talking with the British army until we know what they are here for.”

There is currently a lull in violence, which Catholics say is only due to the fact that the RUC has stopped attacking them.

RTE Reporters Tom McCaughren and Pat Sweeney on the arrival of British troops in Derry.

https://www.rte.ie/archives/exhibitions/1042-northern-ireland-1969/1048-august-1969/320426-british-troops-in-bogside-derry/