A postage stamp that started a war

Stamps depicting a map of the Gran Chaco were issued in 1932

Both Bolivia and Paraguay had conflicting territorial claims over the region known as "Gran Chaco." The dispute escalated in 1931 when Bolivia issued postage stamps featuring a map where the region was labeled as "Chaco Boliviano." In response, Paraguay issued its own stamp in 1932, featuring the slogan "Has been, is, and will be," with the region labeled as "Chaco Paraguayo." The issuance of these stamps symbolized the intensifying tensions between the two countries.

Subsequently, a full-fledged war erupted over the disputed territory. The conflict resulted in significant casualties on both sides, and Bolivia's economy suffered severe devastation as a result. The war continued for several years until a resolution was reached during the Conference of American States held in Buenos Aires from 1938 to 1939.

At the conference, negotiations took place, and an agreement was reached to settle the dispute. Ultimately, the territory was awarded to Paraguay, thereby bringing an end to the war. The outcome of the conference was significant in determining the ownership of the Gran Chaco region and establishing Paraguay's sovereignty over it.

Source: http://www.groseducationalmedia.ca 
http://www.aps.gen.nz/149/chaco_war.html