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 Theater of Operations.
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. Michael D. Swords writes:
During WWII, the foo fighter experiences of [Allied] pilots were taken very seriously. Accounts of these cases were presented to heavyweight scientists, such as David Griggs, Luis Alvarez and H.P. Robertson. The phenomenon was never explained. Most of the information about the issue has never been released by military intelligence.
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The nonsense word "foo" emerged in popular culture during the early 1930s, it was first used by cartoonist Bill Holman who peppered his Smokey Stover fireman cartoon strips with "foo" signs and puns. Holman claimed to have found the word on the bottom of a Chinese figurine. It was part of service culture by World War II and is thought to have led to the backronym FUBAR. By 1944, the term "foo fighter" was used by radar operators to describe a spurious or dubious trace.
The term foo was borrowed from Bill Holman's Smokey Stover by a radar operator in the 415th Night Fighter Squadron, Donald J. Meiers, who it is agreed by most 415th members gave the foo fighters their name. Don was from Chicago and was an avid reader of Bill Holman's strip which was run daily in the Chicago Tribune. Smokey Stover's catch phrase was "where there's foo, there's fire" and this was possibly derived from the French word for fire, "le feu". In a mission debriefing on the evening November 27, 1944, Fritz Ringwald, the unit's S-2 Intelligence Officer, stated that Don Meiers and Ed Schleuter had sighted a red ball of fire that appeared to chase them through a variety of high-speed maneuvers. Fritz said that Don was extremely agitated and had a copy of the comic strip tucked in his back pocket. He pulled it out and slammed it down on Fritz's desk and said, "... it was another one of those fuckin' foo fighters!" and stormed out of the debriefing room. However, in a Channel 4 documentary aired 3rd June 2011, reporter Nick Cook showed an RAF pilot's report, obtained from RAF archives, reporting a UFO incident with a similar red ball of fire on a bombing mission over Germany, but dated 1942 and taken with fact that the term was already in use by radar operators in 1944, must raise some doubt as to the origin of the term 
According to Fritz Ringwald, because of the lack of a better name, it stuck. And this was originally what the men of the 415th started calling these incidents: "Fuckin' Foo Fighters." In December 1944, a press correspondent from the Associated Press in Paris, Bob Wilson, was sent to the 415th at their base outside of Dijon, France to investigate this story. It was at this time that the term was cleaned up to just foo fighters. The unit commander, Capt. Harold Augsperger, also decided to shorten the term to foo fighters in the unit's historical data.
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."
Foo fighters were reported on many occasions from around the world; a few examples are noted below.
"Pilots do not have sufficient information about phenomena of disorientation, and, as a corollary, are given considerable disorganized, incomplete, and inaccurate information. They are largely dependent upon their own experience, which must supplement and interpret the traditions about 'vertigo' which are passed on to them. When a concept thus grows out of anecdotes cemented together with practical necessity, it is bound to acquire elements of mystery. So far as 'vertigo' is concerned, no one really knows more than a small part of the facts, but a great deal of the peril. Since aviators are not skilled observers of human behavior, they usually have only the vaguest understanding of their own feelings. Like other naive persons, therefore, they have simply adopted a term to cover a multitude of otherwise inexplicable events."
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