The term foo fighter was used by Allied aircraft pilots in World War II to describe various UFOs or mysterious aerial phenomena seen in the skies over both the European and Pacific theaters of operations.
Though “foo fighter” initially described a type of UFO reported and named by the U.S. 415th Night Fighter Squadron, the term was also commonly used to mean any UFO sighting from that period.
Basically they were balls of light which followed and hovered around planes of all nationalities.
The first sightings occurred in November 1944, when pilots flying over Germany reported seeing fast-moving round glowing objects following their aircraft. The objects were variously described as fiery, and glowing red, white, or orange.
Some pilots reported that they seemed to toy with the aircraft, making wild turns before simply vanishing.
They reported that the objects flew formation with their aircraft and behaved as if under intelligent control, but never displayed hostile behavior. However, they could not be outmaneuvered or shot down.