WWII War/Propaganda Forgeries
During WWII, the major European combatants counterfeited stamps of their opponents as part of multiple propaganda campaigns.
The campaigns generally served one of two purposes. Some were designed to inject subversive materials into the enemy’s postal system, and for this purpose needed accurate forgeries that were virtually indistinguishable from the originals. Others were designed as obvious propaganda, and as such, aren’t designed to be mistaken for the originals.
In general, these efforts were largely unsuccessful. However they do make for interesting philatelic history.
For a detailed description of these efforts, see Propaganda and Espionage Philately, by SGM Herbert A. Friedman (Ret.).