Cases of mistaken identity?
Known aerial phenomenon?
Or visitors from another planet observing mans self destruction?
The Foo Fighter phenomena was taken very seriously at the time of the sightings during WW2, reports were studied by very well known scientists such as H.P.Robertson and others however no information has ever been released that satisfactorily explains what the phenomenon was.
A popular comic strip at the time by cartoonist Bill Hollman featured a character known as Smokey Stover, this characters favourite saying was " Where there's foo there's fire " as many of the objects spotted looked fiery in nature, the words Foo Fighter seemed quite an apt name to describe them.
Most descriptions of Foo Fighters describe glowing red or orange balls that would follow the plane performing outrageous maneuvers that were just not possible in any allied plane, this led initially into the belief that the objects may have been a German secret weapon, however after the war it emerged that the Axis pilots were also seeing similar objects during their missions. Strangely not only were these objects seen on numerous occasions they were also captured on radar, as reported by radio operator Andrew. V. Ambrose "I had frequently picked up a target on the radar screen that appeared to be a conventional aircraft," he said. "But... upon being tracked [it] would accelerate to a fantastic speed, which made it impossible to set a rate on and even more difficult to identify. So we referred to them as 'ghosts'... I have always been puzzled by the occurrence of these sightings I have personally made on radar.". So it certainly seems that in many instances that the Foo Fighter sightings definitely had a physical substance and were most definitely not just optical illusions.
UFOs are often spotted by commercial airline pilots today but the sheer amount of Foo Fighters spotted during WW2 by both sides is quite unbelievable, one explanation could be that they were an electromagnetic phenomenon something akin to St Elmos fire, indeed in all case sightings the objects never appeared to impose a physical threat. One B29 gunner claimed to have fired and hit a Foo Fighter, with it breaking up into smaller pieces before falling to the ground and setting fire to some buildings below, another possible explanation could be they were some sort of dazzle effect caused by flak fire, this could explain certain sightings if they were near land but not the sightings that would occur over sea, no definitive answer has ever been given to what Foo Fighters were and to this day they remain in the file marked unexplained.