"The Troubles" lasted a long time and it simply took a long time for the violence to be contained and reduced to an acceptable level. Violence came from both sides of the communal divide and sadly some collusion by local security forces. This was mainly the UDR and one UK report cited 5-15% had strong links with Protestant paramilitaries; collusion was rarely found in the RUC and to my knowledge not in the regular Army.
The intelligence effort appears to have taken longer to get right and still causes controversy e.g. Omagh bombing and whether GCHQ helped (Try: http://news.bbc.co.uk/panorama/hi/fr...00/7868236.stm. )
The UDR initially had a significant Catholic minority for a few years, now often overlooked, but by its end it was 99% Protestant. Through out it was mainly a mix of full-time and part-time, locally recruited soldiers - with a cadre of regular UK Army officers and NCOs. IIRC their main role in later years was in the rural areas, the RUC handled the urban areas and rarely had UDR support. Being in the UDR could not be easily hidden in the rural areas and they paid a very high price, often even when retired or had left the UDR (264 dead).
I suspect the UDR initially was a quick fix and politically motivated - to keep loyalist support in uniform. Their existence after many years enabled the UK Army to reduce its numbers and extended "Ulsterisation" in policing to the military in the mid-1970s it was eleven battalions strong and 57,000 served in its history. I am not aware of how vital a role the UDR played, nor can I readily locate an expert comment.
My judgement or comment reflects the fact that the main focus throughout "The Troubles" was that the "hardcore" Republican areas were urban, where the UK Army handed primacy over to the RUC in the mid-1970's, so the UDR's value was not crucial IMHO.
In the rural areas the UDR had an active role long after primacy was handed to the RUC, but the regular UK Army dominated intelligence and surveillance roles.
Many other factors had a part in ending "The Troubles".
Late addition: On my bookshelf and relied upon for much of my response is this book 'Testimony to Courage - the Regimental History of the Ulster Defence Regiment 1969 - 1992', John Potter, Pen & Sword Books Ltd, 2001.
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