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. Formally reported from November 1944 onwards, witnesses often assumed that the foo fighters were secret weapons employed by the enemy, but they remained unidentified post-war and were reported by both Allied and Axis forces.
The first sightings occurred in November 1944, when pilots flying over Germany by night 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 described them as resembling Christmas tree lights and reported that they seemed to toy with the aircraft, making wild turns before simply vanishing. Pilots and aircrew 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. The phenomenon was so widespread that the lights earned a name - in the European Theater of Operations they were often called "kraut fireballs" but for the most part called "Foo-Fighters". The military took the sightings seriously, suspecting that the mysterious sightings might be secret German weapons, but further investigation revealed that German and Japanese pilots had reported similar sightings.
In its 15 January 1945 edition Time magazine carried a story entitled "Foo-Fighter", in which it reported that the "balls of fire" had been following USAAF night fighters for over a month, and that the pilots had named it the "Foo-Fighter". According to Time, descriptions of the phenomena varied, but the pilots agreed that the mysterious lights followed their aircraft closely at high speed. Some scientists at the time rationalized the sightings as an illusion probably caused by afterimages of dazzle caused by flak bursts, while others suggested St. Elmo's Fire as an explanation.
The "balls of fire" phenomenon reported from the Pacific Theater of Operations differed somewhat from the Foo Fighters reported from Europe; the "ball of fire" resembled a large burning sphere which "just hung in the sky", though it was reported to sometimes follow aircraft. On one occasion, the gunner of a B-29 aircraft managed to hit one with gunfire, causing it to break up into several large pieces which fell on buildings below and set them on fire. As with the European Foo Fighters, no aircraft was reported as having been attacked by a "ball of fire" The postwar Robertson Panel cited Foo Fighter reports, noting that their behavior did not appear to be threatening, and mentioned possible explanations, for instance that they were electrostatic phenomena similar to St. Elmo's Fire, electromagnetic phenomena, or simply reflections of light from ice crystals. The Panel's report suggested that "If the term "flying saucers" had been popular in 1943-1945, these objects would have been so labeled."