The War of the Triple Alliance

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The Paragauy War, commonly known as the War of the Triple Alliance, which was waged from 1865 until 1870, was the largest war ever fought in South America. It was in the immediate aftermath of the colonial period and the international borders between the new countries were ill-defined leading to a lot of diplomatic tension.

In that cauldron in Uruguay two factions were vying for control. The Brazilians backed one of them and Francisco Solano Lopez, the lunatic dictator of Paraguay, took offense to that figuring that if anybody had a right to meddle in Uruguay's affairs it was him and not the Brazilians.

Eventually this all boiled over when Lopez invaded and annexed Mato Grosso, a Brazilian province. He then foolishly declared war on Argentina so he could march through its territory to attack Uruguay. To add to his stupidity, he declared that he had annexed two Argentinian territories as well. As result, Argentina and Uruguay both declared war on Paraguay with Brazil secretly joining the alliance. 

The war proper started with a naval battle on the Parana River, where the Brazilian fleet soundly trounced the Paraguayans. Lopez's ground prong of his invasion fared no better, in short order almost all of his troops were either killed or captured. A few months later the Triple Alliance crossed the Paranan and invaded Paraguay. 

By 1869 they had captured Asuncion, Paraguay's capitol, and destroyed Lopez's army. Lopez himself withdrew into the jungle and tried to maintain resistance. As the above linked article says:

That summer, tipped off as to Lopez’s whereabouts, d’Eu crossed the border with 27,000 fresh, combat ready troops and moved in for the kill. On August 11, they surrounded Piribebuy and proceeded to subject the inhabitants to a general bombardment. At 8 am the following day, the task force began making its way up the heights. The Paraguayan defenders opened fire, but they soon ran out of ammunition and had to resort to throwing stones at their attackers. By noon, the Allied army entered Piribebuy over the lifeless bodies of 700 soldiers. D’Eu and his army had killed them all.

All but Lopez. Once again, he had used the destruction of women and children to cover his escape. This time he headed northeast toward Bolivia, one of the few countries in South America that was not at war with him. As El Supremo and a cohort of loyalists made their way into the rain forest, the rear guard charged with covering his retreat was quickly mowed down by d’Eu and the army of avenging nations. On March 1, 1870, the Allies finally caught up with Lopez as he was preparing to cross the Aquidaban River, not far from the Bolivian border. While his henchmen died in a last stand on the banks of the river, Lopez, true to form, met his ignoble end in the water. Wounded by a lance, he thrashed into the shallows, where a bullet smashed into the back of his head as he tried to swim across the river.

The war was a complete disaster for Paraguay. They had much of their territory annexed, but worse is their casualty figures: variously estimated be between 7% on the low end, to 69% of the total population. At war's end there were only 28,746 adult men left. The effects of those casualties and that greatly unbalanced sex ratio echo in Paraguay to this day.