This article is from the Foo Fighters FAQ, by Ben Rogersbenson@unix3.netaxs.com with numerous contributions by others.
Near the end of WWII, the U.S. Air Force patrolling German airspace
encountered highly maneuverable balls of light in the area between
Hagenau in Alsace-Lorraine and Neustadt an der Weinstrasse in the
Rhine Valley. These unidentified flying objects came to be referred to
as "Foo Fighters", or "Kraut Balls" by those who believed the objects
were a secret German weapon.