Alain Resnais's 1956 thirty-two-minute confrontation with Nazi concentration camps. Still the most devastating short film ever made.
From Wikipedia
Nacht und Nebel, meaning Night and Fog, also known as the Night and Fog Decree, was a directive issued by Adolf Hitler on 7 December 1941 targeting political activists and resistance "helpers" in the territories occupied by Nazi Germany during World War II, who were to be imprisoned, executed, or made to disappear into the "night and fog" of the Reich. The family and the population remained uncertain as to the fate or whereabouts of the alleged offender against the Nazi occupation power. Victims who disappeared in these clandestine actions were often never heard from again.