In 1864, Paraguay, which had been a reclusive, insular state in the fifty years or so since independence from Spain, used a border skirmish between Brazil and Uruguay as an excuse to declare war on Brazil: their true objective was to gain a port on the Atlantic Ocean. A few weeks later, they declared war on Argentina as well, because the Argentines refused to let them pass through their territory. Uruguay, under Brazilian influence, joined the war as an ally of Brazil and Argentina.
At first, Paraguay looked as if it would win the war. During its years of isolation, Paraguay had militarized, and had a military force larger than that of its three opponents combined. Brazilian naval superiority, however, eventually proved decisive and the balance of the war eventually shifted. The three allies prevailed.
Paraguay was left devastated by the war. Most historians agree that probably as much as half of Paraguay’s pre-war population of around 550,000 perished in the war, including as much as 90% of the men. The country was utterly crushed: the only thing that prevented Brazil from incorporating Paraguay into their territory was the fact that Argentina would not allow it.
Repercussions from the bloodiest war in South America since the conquest of the Inca lasted for decades. The Brazilians did not leave Paraguay until 1876. Certain border disputes were not fully settled until 1878. Paraguay owed a war debt to Brazil and Argentina until 1943, and certain cultural artifacts taken during the war were not returned to Paraguay until 1975.