 To ensure authenticity, director Milestone, Lewis (qv) instructed  the studio to try to find out if there were any German Army veterans  living in the Los Angeles area, so he could have them authenticate  German uniforms, equipment, etc. So many of them were found that  Milestone cast a lot of them as German officers in the film, and had  them drill the extras playing German troops. (The scene where they  are laying communication wire in the forward trenches was led by a  former German soldier whose job during the war was to do exactly  that.)