BBC Northern Ireland London correspondent
Top secret files on a Nazi plan to invade Northern Ireland during the second World War are being made public on Friday for the first time.
The documents, which were gathered by the intelligence service MI5, detail how a German spy hatched a plot with an IRA commander in Dublin, which became known as 'Plan Kathleen'.
If it had ever been put into action, 50,000 Nazi troops would have invaded Northern Ireland with assistance from IRA brigades from the south.
The files released contain details of 'Plan Kathleen'
The two plotters were Hermann Goertz, a German spy who had already been convicted in England for espionage in the 1930s and deported to Germany, and Stephen Held - a leading IRA activist in Dublin.
It is thought that the two originally met in Frankfurt in 1940 and that Held convinced Goertz that he had 5,000 IRA men ready to fight alongside the Nazis if they invaded Northern Ireland - so long as the Germans provided arms.
Originally, Held was believed to be the sole brain behind the idea but this latest release of MI5 files shows Goertz's handwriting on the original plans.
The files which have been released set out the detail of 'Plan Kathleen'.
The plan was to land 50,000 German troops at a number of different places, including Coleraine, Larne, Londonderry and Sligo.
These troops would then be met and assisted by IRA brigades moving to meet them from Ballyshannon and Dundalk.
Howard Davies, an archivist at the Public Record Office for the last 18 years said the Nazi's were hoping to stir up trouble for Britain however they could.
"Firstly, obviously, they're looking to hit British economic targets and the North," he said.
"Belfast was very important to Britain at the time. They were looking to threaten the Atlantic convoy routes - and they are looking to tie down British troops. So they have got three real strategies here ."
It is generally accepted that the plan was never taken too seriously by Nazi commanders in Germany.
But in 1940 Goertz was dispatched to Northern Ireland to conduct a recce.
German spy Hermann Goertz was dispatched to Northern Ireland in 1941
He parachuted in in the dead of night - only to find he had landed in County Westmeath in the midlands.
Both he and Stephen Held were arrested shortly afterwards and documents relating to 'Plan Kathleen' were seized at Held's Dublin home.
Those documents are now part of the MI5 files which have been released.
The files hold correspondence on 'Plan Kathleen' between the authorities in London, Dublin and Belfast as well as photographs of the German spy Hermann Goertz.
There are even maps of Northern Ireland - taken from Held's house - with various possibilities for the invasion marked out on them.
The plot has been known about for some time.
But from Friday, anyone will be able to visit the National Archives at the Public Record Office in London, to see for themselves the once top secret evidence that it really did exist.