Feldpost is the German military mail service.  It traces its roots back 18th century in the Kingdom of Prussia, and has existed ever since in different forms and shapes.

During 1937-39, the German Wehrmacht had a military mailing service that provided free postal services within Germany.  With the outbreak of WWII, the service was greatly expanded to provide service to the millions of German troops scattered over Europe, Asia, and Africa. 

Over the course of the war, minor changes to the service would be necessitated by the scope of the German conquests, most notably the frequent use of local provisional feldpost stamps in areas isolated due to the detiorating military situation.