Stamp Wars
Believe it or not, wars between countries have been fought using postage stamps! Although it has been quite quiet on the stamp front lately, we only need to go back to the year 1932 to see a real stamp war.
The war took place in an flammable South America, more specifically between Paraguay and Bolivia.
What exactly was it about?
The conflict arose over an area called Gran Chaco, which both countries claimed as their own. Because oil was found nearby, it was assumed that the Chaco region was rich in oil. Originally, the conflict was fought between the oil companies Royal Dutch Shell, which represented the interests of Paraguay, and Standard Oil, which claimed the area for Bolivia.Postage stamps as a coat of arms
The first weapon both countries used in this war was postage stamps. On these stamps, Gran Chaco was clearly included within the national borders of both countries.
This relatively peaceful start to the conflict between Paraguay and Bolivia eventually escalated into an armed conflict that lasted from 1932 to 1935. Ultimately, Paraguay won this war, and in 1938 a treaty was signed whereby it received three-quarters of the territory and Bolivia the remaining part. There indeed turned out to be a lot of oil and gas in the ground, but ironically, the majority of it could not be extracted until this century. An estimated 100.000 people died in the Chaco War.
The First Stamp War
As far as is known, the first philatelic event took place in 1896. Who were the warring parties? Venezuela and Great Britain. What was the reason? Literally a handful of stamps issued to commemorate the death of General Miranda 80 years earlier. These stamps depicted a map in which a large part of British Guiana lies within the borders of Venezuela.
You can find more information about the Venezuelan crisis of 1895 at here (in English). 
Nice story. I always thought that the only thing that applied was: football is war, but apparently so is philately.
It was once almost war between Israel and Yemen over a postage stamp.
In 1967, the Kingdom of Yemen issued a series (Michel 420-426) depicting the visit of the Queen of Sheba to King Solomon of Israel. One stamp from the series (Michel XII) was not issued. That stamp depicts the Western Wall with Arabs and a dromedary in the foreground. Israel threatened bombardments if this stamp were issued.
Interesting addition, Lars! I didn't know that one.
You're welcome! I think there was also a dispute once between Haiti and the Dominican Republic over a shifted border on stamps, but I don't remember the details.
On Google, under the term 'postal war', numerous images of stamps and franked letters with background information can be found that supplement and further complete this article.
And how was it with the Falkland Islands, between Argentina and England?
Argentina has issued stamps dedicated to the “Islas Malvinas” on several occasions, the last time in 2012, exactly 30 years after the war! I have never read anywhere whether these and previous issues are accepted in the UK or not.
An article about the Chaco War and other stamps concerning Bolivia's border conflicts was published in 'The New Cartophilatelist 32' (January 2011).
http://www.mapsonstamps.org/publications/tcnp-tables-of-contents/issues-29-32-2010/
Due to the issuance of stamps featuring maps by the Dominican Republic in 1900 (Mi 84-92), a war nearly broke out because part of Haiti is depicted as part of the Dominican Republic on those stamps.
Incidentally, there is also a site with errors on stamps featuring maps:
http://danstopicals.com/errorsindex.htm