In 1940, Fleischhacker also joined both the
Nazi Party and the
Waffen-SS. Before long, he saw service with the
SS Race and Settlement Main Office. Following
the invasion of
Poland, he was sent to
Litzmannstadt as part of this group in order to perform measurements on
ethnic Germans and determine whether they were suitable for resettlement programmes in the east or simply for
forced labour. In 1942, Fleischhacker was, along with
Heinrich Rübel, chosen by
Bruno Beger to work with him in an SS project to determine the racial type applicable to the
Mountain Jews of the
Caucasus, a group that did not readily fit the Nazi's existing ethnic paradigms. Fleischhacker was chosen for this task due to a thesis he was preparing at the time dealing with skin color amongst
Jews. The following year he worked again with Beger, this time at
Auschwitz concentration camp where again his duty was to measure physical features of the inmates in order to determine to which race they belonged. He took measurements of the persons selected for the
Jewish skull collection, who were sent from Auschwitz to Natzweiler-Struthof concentration camp to be gassed. ==Post-war trial and work==