Great South American War

The Great South American War was a conflict between Bolivia and Paraguay over the control of a great part of the Gran Chaco region of South America which extended in a war between Brazil and Argentina and led to the creation of the Union of La Plata.

Origins
Though the region of the Gran Chaco 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). Furthermore, the discovery of oil in the Andean foothills sparked speculation that the Chaco itself might be a rich source of petroleum.

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, however, Paraguay had begun to colonize the region; indeed, both Paraguayan and Argentinian planters already bred cattle and exploited quebracho woods in the area. Paraguay had lost almost half of its territory to Brazil and Argentina in the War of the Triple Alliance in 1870 and was not prepared to see what it perceived as its last chance for a viable economy fall victim to Bolivia.

Initial conflict
Border skirmishes throughout the late 1920s culminated in an all-out war in 1932, after the Bolivian army attacked a Paraguayan garrison at Lake Pitiantuta in June. Paraguay quickly mobilized its troops but Bolivia was able to overwhelm their defences. Paraguayan troops were repeatedly defeated and Bolivia overran most of the country by the beginning of 1933.

Argentinian intervention
The Bolivian occupation of Paraguay prompted Argentinean intervention in 1933, ostensibly over the attempted Bolivian annexation of Paraguay. In a quick campaign Argentinean troops liberated Paraguay and settled in for the winter. In 1934, Argentina renewed their offensive and scored several great victories against the Bolivian forces. At this point Bolivia seemed totally defeated.

Brazilian intervention
However, in another twist of the war, Bolivia's hope was rekindled, as Brazil chose this time to intervene on their side. Unfortunately, however, the Brazilians focused the thrust of their offensives towards the La Plata River, leaving the Bolivians to look after their own defences, and this error of judgement lead to a humiliating final defeat of Bolivia at the hands of Argentina in late 1934. Despite a successful initial advance towards the La Plata River (where the tiny nation of Uruguay was overran in the process), the Brazilian advance was halted as the war got bogged down in trench systems. Brazil was unable to withstand the pressure when Argentinean forces counterattacked in 1935, and the Brazilian troops were forced on a great retreat. With Argentinean forces poised to take Curitiba and her own armed forces in disarray, Brazil sued for peace.

Aftermath
Following its defeat in 1934, Bolivia was forced to give to Paraguay the entire Gran Chaco region. In the peace that followed the defeat of Brazil in 1935, the countries of Paraguay (with the entire Gran Chaco region) and Uruguay were "federated" with Argentina to create the Union of La Plata.