With undocumented belief that an arm stretched forward with palm facing down was an ancient Roman military salutation, Italian Fascists first adopted it, then the Nazis. (Ironically, the original American Pledge of Allegiance was similar until changed to hand over the heart).
The Nazi salute Deutscher Gruß (German Salute)) was reminiscent of a social greeting used among medieval German civilians. Although often used in Nazi rallies, the Deutscher Gruß, did not become mandatory in any service until the failed attempt on Hitler’s life in July 1944.