[Hugh]

That sounds like a stamp issued by Bolivia in 1928 (Sc. 191) showing the Gran Chaco Region ... the source of a long-standing dispute between Bolivia and Paraguay. 

Booth countries issued stamps between 1924 and 1932 in which they portrayed their imagined (and disputed) bordersr. 

   
Sourced: Wikipedia / Public Domanin Image

As you say, when the 1928 stamp was issued showing Chaco as part of Bolivia, the people of Paraguay were not amused. A few years later, the two counties went to war. The Chaco War (1932-35) is known in Spanish as La Guerra del la Sed (The War of Thirst) as the troops fought in a semi-dessert. 

Of course, it was really about money ... control of what were seen as a region rich in petroleum. Royal Dutch Shell supported Paraguay and Standard Oil supported Bolivia. The high command of Bolivia included former WW1 German officers and Paraguay engaged the services of 80 former White Russian officers who fought in the Russian Civil War. Italy also provided them with advisors who provided military training. US politician Huey Long was strongly in support of Paraguay. At one point in the war, Paraguay named a captured for after him .. Fort Long. 

It was the bloodiest war in South America in the 20th century. When a ceasfire was negotiated, Paraguay controlled most of the region. The war formally ended with the Chaco Peace Treat in July 1938 when the two countries agreed to divide the region. Paraguay was awarded about 75% of what was, essentially, a dessert.

It took a long time to find it but both countries did end up with significant oil and gas fields. 

I love the way there is a story behind every stamp. 

Cheers, Hugh