In 1941 Lieutenant Archibald David Stirling, a supplementary reserve officer of the Scots Guards, along with five co-founders created the Special Air Services, know then as the SAS Brigade. It was formed as a volunteer desert raiding airborne force unit who secretly fought far behind enemy lines. Additionally, there were three units created within the SAS; the Special Boat Service (SBS), the Special Reconnaissance Regiment (SRR), and the Special Forces Support Group (SFSG).
The SAS became a key entity during the Second World War, as the Regiment’s motto is “Who Dares Wins.” World War II involved the mobilization of over 100 million military personnel, to include the SAS, making it the most widespread war in history. It resulted in the death of over 60 million people, making it the deadliest conflict in human history. More information at this site HERE
Another account of the SAS in five segments, HERE