The Republic of Ireland considered joining the Allies during World War II, according to a new book.
German troops were to be tricked into painting their feet blue
According to the wartime journal of MI5's Director of Counter-Espionage, Guy Liddell, an agent wrote to Eamon De Valera in September 1940.
The Irish prime minister wrote back to say he was "intrigued" by the idea.
Among the other wartime secrets in the book by Nigel West is that British agents painted one foot blue in an attempt to fool German spies.
This was part of a bizarre trap laid for Nazi parachutists.
It was thought Mr Liddell's adversary, Admiral Canaris, was plotting to drop dozens of German agents into Britain to prepare for the invasion of the UK.
So Mr Liddell came up with the idea of passing on certain disinformation through double agents working in Germany.
This led the Germans to believe that British soldiers routinely painted one foot blue so they could be identified as being genuine.
The idea was that when the Germans parachuted their own agents into England with one foot painted blue the Home Guard would be able to identify them as spies and either arrest or shoot them on sight.
Guy Liddell was Britain's top spymaster during the war
The book also gives extraordinary details of the workings - and failings - of British intelligence during World War II.
Mr Liddell was Britain's top spymaster at the time and his wartime journals have been classified until very recently.
The Fifth Man
The book also suggests that the "unidentified man in the Cabinet Secretariat" was John Cairncross - the so-called Fifth Man of the Cambridge spy ring which also included Burgess, Maclean, Philby and Blunt.
Although a mole is identified in Liddell's diaries, it was not until 1951 that the security services identified him; and as recently as October 1990, John Cairncross was insisting to the BBC that he was not the Fifth Man.
As Director of B Division, Mr Liddell supervised all the major pre-war and wartime espionage investigations.
He was in charge of maintaining a watch on suspected Fifth Columnists.
As for the attempts to encourage Ireland to join the Allies, these were ultimately unsuccessful. The Republic remained neutral until the end of the war.
Mr West is perhaps better known as the former Conservative MP Rupert Allason.