Disclaimer: I have not seen very many films.
In the film "die Bruecke" (1959) there was a split-second scene shot from behind of the silhouettes of two helmeted heads inclined towards each other as the sixteen year old boys spring back up from the safety of their foxhole to man a machine gun. The scene was very reminescent of numerous still shots found in history books and I found it very disturbing. That image has stayed with me throughout the years and even now as I type this my stomache is in a knot as I ask myself: What in God's name were those children doing in a foxhole?