Psywar.org posted a deep history of propaganda and espionage stamps. The article focuses on black propaganda, "items produced secretly by one nation to be used against another," and features a slew of examples. Seen here is a Hitler stamp parody used as propaganda by the United States Office of Strategic Services during World War II. From "Propaganda and Espionage Philately" by SGM Herbert A. Friedman (Ret.):
Perhaps we should take a moment to define the difference between a propaganda parody and an espionage forgery. In the former, one government will take the stamp of an enemy or occupied government and change the stamp in such a way as to make a political statement and perhaps cast aspersions at and ridicule enemy leaders or occupation forces. These stamps are designed to be recognized for what they are, an attack on the enemy. In the case of the espionage forgery, one government has produced a stamp that is a perfect imitation of the enemy stamp to be used to mail propaganda or instructions to people in the enemy or occupied country. These stamps are part of a secret operation and not meant to be recognized.