A guy I worked with used to tell me stories of his Croatian American father working with the resistance behind the lines in the former Yugoslavia during the war. Sounded like some very brutal bloody work. There was an SAS connection there but I never could quite follow whether they were simply working with or actually for the SAS.
‘With’ probably, he most likely would have been OSS (Sterling Hayden being the most famous OSS agent in Yugoslavia). The British used the SOE.
The SAS were used in that area, both mainland and the few Yugoslav islands, but it was the SOE and OSS who were the main allied support.