Postage stamps have long been used for political/propaganda purposes. Some stamps have been issued by countries to show claim to land that another country had over the same land.
In the early 1930s, Paraguay and Bolivia got into a war over claims to the same piece of land. Prior to the war, each country issued stamps depicting the area, and declaring it their own.
Here's a link that shows two of the stamps and a brief history on the war.
stamp that started a war
 
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