Excerpts from Books and Wikipedia
"The Paraguayans fought again, and triumphed, in a war against Bolivia, the Chaco War (1932–35). The Chaco is a desolate region of suffocating heat, alternating flood with drought, west of the Paraguay River. Competing claims to the region had long put Paraguayans and Bolivians at odds, but the claims suddenly flared with the discovery of oil there in the 1920s. Soldiers from the Bolivian highlands suffered particularly in the difficult and unfamiliar Chaco environment. Paraguayan victory in the Chaco War doubled the national territory and worked wonders for national pride. The Chaco War turned out to be the only major war fought among American countries in the 1900s." [Born in Blood & Fire, 4th Ed., p. 190]