The Chaco War (1932–1935) was fought between Bolivia and Paraguay over control of the northern part of the Gran Chaco region (the Chaco Boreal) of South America, which was incorrectly thought to be rich in oil. It is also known as La Guerra de la Sed or "War of Thirst". It was the bloodiest military conflict fought in South America during the 20th century.
Though the region was sparsely populated, control of the Paraguay River running through it would have given one of the two landlocked countries access to the Atlantic Ocean. This was especially important to Bolivia, which had lost its Pacific Ocean coast to Chile in the War of the Pacific (1883).
In international arbitration, Bolivia argued that the region had been part of the original Spanish colonial province of Moxos and Chiquitos to which Bolivia was heir. Meanwhile, Paraguay had begun to colonize the region. Indeed, both Paraguayan and Argentinian planters already bred cattle and exploited quebracho woods in the area, while the small indigenous population of Guaraní-speaking tribes was related to that country's own Guaraní heritage.
Control of natural resources
Furthermore, the discovery of oil in the Andean foothills sparked speculation that the Chaco itself might be a rich source of petroleum. Foreign oil companies were involved in the exploration: companies mainly descended from Standard Oil backed Bolivia, while Shell Oil supported Paraguay. Standard was already producing oil from wells in the high hills of eastern Bolivia, around Villa Montes.
Paraguay had lost almost half of its territory to Brazil and Argentina in the War of the Triple Alliance and was not prepared to see what it was perceived as its last chance for a viable economy fall victim to Bolivia.
Border skirmishes throughout the late 1920s culminated in an all-out war in 1932, after the Bolivian army, following orders of President Daniel Salamanca, attacked a Paraguayan garrison at Lake Pitiantuta in June. They later occupied another garrison further south, called Fortín Boquerón. This stronghold would later become the scene of one of the bloodiest and protracted battles of this war, when some 600 Bolivian soldiers resisted a 22-day siege, against 14,000 Paraguayan troops before surrendering (7-29 September 1932).
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