Since 1944 Allied pilots were reporting over Europe and Pacific sightings of unknown objects, usually in shape of a glowing ball, that were following their planes and even with high speed manuevers they were unable to shake them off. Of course the most logical conclusion was that they are some sort of unknown weapon used by the Germans and Japanese, hence the names of those objects: "kraut fireballs" or much more popular "foo fighters". But the stange thing is that German and Japanese pilots were reporting similar sightings and they also were convinced that it is a secret weapon of Allies.
One of the first sighting took place in November 1944 when night fighters over Europe reported that objects glowing red, white or orange light were following them at high speed. On 13 December 1944, the Supreme Headquarters Allied Expeditionary Force in Paris issued a press release, which was featured in the New York Times the next day, officially describing the phenomenon as a "new German weapon". Follow-up stories, using the term "Foo Fighters", appeared in several other newspapers, including the New York Herald Tribune and the British Daily Telegraph.
The same phenomenon reported over Pacific was a little bit different, usually the objects 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 bomber 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. There was speculation that the phenomena could be related to the Japanese fire balloons’ campaign (Japanese were sending high altitude balloons over Pacific hoping they will start fires in United States).
There is no report that any of the foo fighters at any time have engaged the plane it was following and in general their behaviour was rather non-threatening.
Of course after the war, especially since the German and Japanese reports were available, it became blindingly obvious that the foo fighters couldn’t be a secret weapon used by any country involved in World War 2. Especially since after the war the sightings were still reported.
The simplest explanation, and often repeated, is some sort of natural phenomenon, but if the reports about the objects fast manuevers and especially the one about hitting it with bullets were true, this would rule out any sort of natural cause.
Can you solve a mystery like this one?