British Prime Minister Margaret Thatcher admitted to Irish officials the UK "got it wrong in 1921" with the Northern Ireland border.
A series of extraordinary admissions between Mrs Thatcher and Taoiseach Dr Garret Fitzgerald have emerged from the 1986 archives released under the 30-year-rule.
Both leaders were in contact throughout 1986 as the UK and Ireland backed the Anglo-Irish Agreement amid furious Unionist and Loyalist opposition to the deal.
The pair met on December 6, 1986 in London - and Mrs Thatcher made a series of blunt admissions in assessing the Irish security situation.
"You (the Republic) haven't the resources to maintain protection on the other side of the border," she said.
"I do feel very depressed at times about the whole situation. The violence has not been defeated. The SDLP have not done what we are expecting them to do."
"However, it is Christmas - and I had better stop feeling depressed."
Dr Fitzgerald praised the RUC for the work they had done in co-operating with gardai and wanted all UDR patrols accompanied by the RUC, but warned that "both forces have a next-to-impossible border to watch".
Mrs Thatcher admitted: "Yes, we got it wrong in 1921."
The meeting concluded with one Irish civil servant noting that "Thatcher went on…including a wistful reference to whether she could continue, in all seriousness, to send young men to their death in Northern Ireland".