The SDLP chief set out the scenario in a five-page letter to Sinn Fein leader Gerry Adams in March 1988, in which he argued for the IRA to abandon its bloodletting.
The letter is one of hundreds of files which have been made newly available from the Irish government archives.
It was given to the University of Ulster’s Conflict Archive on the Internet (CAIN) which published them last week.
In the letter, Mr Hume set out the damage done in terms of lives and property by the “Provisional republican movement”, adding: “The people who have suffered most and the areas who have suffered most are the very people and areas that are represented by either SDLP or Sinn Fein”.
He went on to say: “It is not an answer to suggest that the British presence is the primary source of our problems, and therefore the cause of all the violence.
“All of us take our own decisions and use our own methods for dealing with that presence.
“We must also take responsibility for those methods and for their consequences, particularly when such consequences can be foreseen.”
He said it was “clear” that the UK government would not accede to demands which were backed up by violence, and wondered whether the “Provisional republican movement” had reached the stage when its “methods have become more sacred than the cause”.
He went on: “Even if, of course, the stated objectives of the IRA were to be achieved in the manner which they have set out, we in the SDLP would argue that would not bring peace to Ireland but would lead to much greater chaos and to permanent division and conflict among our people.”
If the UK announced its intention to withdraw from Northern Ireland, then it would leave a “vacuum”, and this “will be filled immediately by force as each section of the community moves to secure its position”.
He said: “In such a vacuum the likelihood is that the British army would become inactive. In the knowledge that their government has decided to withdraw all responsibility, does anyone think that soldiers would be prepared to risk their lives?
“Each section of the community would seize its own territory and we would have a Cyprus/Lebanon style formula for permanent division and bloodshed.”
This was a reference to the long-running power struggle in Lebanon – centred in large part on its capital Beirut – which had pitted Christian, Muslim, and other ‘sides’ against one another since the mid-1970s.
The mention of Cyprus refers to decades of ongoing tension between Greeks and Turks on the island, resulting in deep ethnic division and violence, and eventually in a seperate Turkish-run state being set up on the northern third of the island.
Both Lebanon and Cyprus continue to suffer from these divisions today.
“What would the 12,000 armed members of the RUC do? What would the 8,000 armed members of the UDR do?
“Is it not likely and natural in the emotionally charged atmosphere that would obtain, and in the absence of any acknowledged authority, that they would simply identify with the community from which most of them come and become its military defenders?
“And what would happen to the Catholic community in such circumstances, particularly in those areas where they are most vulnerable?”
He said “the political road is the only one that will ensure that there is lasting peace in Ireland”, which can only come via some kind of agreement between unionists and nationalists.
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