Creating mayhem behind enemy lines, the SAS's elite brand of derring-do played an integral part in defeating Nazi Germany.
Yet some wartime military chiefs dismissed their clandestine tactics as "just not cricket" and tried to get the fighting unit disbanded - before Winston Churchill intervened.
Archive material opened up for the first time in 75 years reveals how Churchill's hard-drinking and overweight son, Randolph, became the SAS's unlikely saviour.
David Stirling, then the young, eccentric pioneer of the unit, pulled off a masterstroke by allowing Randolph to observe some of the early covert activities in the Middle East.
"He is one of the few people who think of war in three-dimensional terms," Randolph wrote to the Prime Minister.
Churchill eventually summoned Stirling to meet him in Cairo and was "bowled over" by the young soldier.
During the meeting, Stirling took a souvenir signature from Churchill and the Commander-in-Chief of the Middle East and typed on the blank page: "Please give the bearer of this note every possible assistance."
Ben Macintyre writes in his newly-published SAS: Rogue Heroes book: "Stirling had no qualms about this blatant forgery: Churchill had become a staunch supporter of the unit and so, he insisted, 'in a sense it was authentic'."
The book, on sale from Thursday, also details how Paddy Mayne, one of the SAS's most famous veterans, destroyed more German aircraft than any other man in the military by running through airfields strapping explosives to planes.
Between 1941 and 1945, the Special Air Service pioneered a form of combat that has since become a central component of modern warfare. It began life as a raiding force in the north African desert, but grew into the most formidable commando unit of the war, and the prototype for special forces across the world, notably the US Delta Force and Navy Seals.
In 1980, an SAS squad successfully stormed the Iranian embassy in London and liberated 25 hostages, killing five of the six hostage-takers in 11 minutes.
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