This concept of gremlins was popularized during World War II among airmen of the UK's RAF units, in particular the men of the high-altitude Photographic Reconnaissance Units (PRU) of RAF Benson, RAF Wick and RAF St Eval. The flight crews blamed gremlins for otherwise inexplicable accidents which sometimes occurred during their flights. Gremlins were also thought at one point to have enemy sympathies, but investigations revealed that enemy aircraft had similar and equally inexplicable mechanical problems. As such, gremlins were portrayed as being equal opportunity tricksters, taking no sides in the conflict, and acting out their mischief from their own self-interest. In reality, the gremlins were a form of "buck passing" or deflecting blame. This led folklorist John Hazen to note that "the gremlin has been looked on as new phenomenon, a product of the machine age – the age of air". Some experts believe this form of "passing the buck" was important to the morale of pilots. Author and historian Marlin Bressi stated, "Gremlins, while imaginary, played a very important role to the airmen of the Royal Air Force. Gremlin tales helped build morale among pilots, which, in turn, helped them repel the Luftwaffe invasion during the Battle of Britain during the summer of 1940. The war may have had a very different outcome if the R.A.F. pilots had lost their morale and allowed Germany's plans for Operation Sea Lion (the planned invasion of the U.K.) to develop. In a way, it could be argued that gremlins, troublesome as they were, ultimately helped the Allies win the war." Bressi also noted: "Morale among the R.A.F. pilots would have suffered if they pointed the finger of blame at each other. It was far better to make the scapegoat a fantastic and comical creature than another member of your own squadron."