Most
ufologists would draw a distinction between foo fighters and conventional "flying saucer"
UFOs. Foo fighters were generally only a few
inches or few
feet across, and were usually
spherical balls of light, not
metallic or discernably
solid in any way. While the foo fighters never
attacked, they did have a penchant for trailing
airplanes at high speeds, chasing them around the
sky.
The assumption was that foo fighters were enemy-controlled reconnaissance drones, or perhaps some kind of psychological weapon. After all, very few foo fighters were seen before 1943, and their sudden prevelance seemed to reek of Axis development. However, captured German and Japanese pilots under interrogation not only denied responsibility for foo fighters... they admitted that they too had been chased around the skies by balls of light!
The earliest reported foo fighter incident took place in the Indian Ocean in September 1941. Two sailors aboard the British troop ship SS Pulaski witnessed a strange green globe trail their ship for over an hour. Apparently foo fighters chased more than planes!
The actual term "foo fighter" hails from the catchphrase of cartoon character Smokey Stover: "where there's foo there's fire" ('foo' being a corruption of the French feu). After the war, foo fighters disappeared from the skies, until the Korean War, when the fiery fliers made a trumphant return. Of course, by that time, many pilots knew them by a different name.
It's unclear exactly how much official foo fighter investigation was carried out during the war, but the minutes of the CIA's 1953 Scientific Advisory Panel on Unidentifying Flying Objects link some rather potent scientific minds to the study of the phenomenon. The overall consensus was that foos were of natural origin, and that remains the prevailing ufological opinion today; likely, they were some mixture of ball lightning, ice crystals, and odd plane reflections and mirages. Still, as long as the mystery remains somewhat open, there will be those who claim that extraterrestrials have a keen interest in wartime nuclear detonations...
By the way, I love the "Everlong" video...