The Paraguayan War, also called the War of the Triple Alliance, was the bloodiest inter-state war in Latin American history, fought between Paraguay and the "Triple Alliance" of Brazil, Argentina, and Uruguay from 1864 to 1870. The war resulted in Paraguay losing two-thirds of its population, including 90% of its male population, as well as 40% of its prewar territories.
Following the South American Wars of Independence, the collapse of Spanish colonial rule left behind unchartered borders between the newly independent South American states. In the Southern Cone alone, Argentina and Uruguay fought Brazil in the Platine War of 1825-1828, Chile and Argentina fought against a Peru-Bolivian Confederation in the War of the Confederation from 1836 to 1839, and Brazil fought Argentina in the Platine War of 1851-1852. As the rest of South America fought a series of border wars, Paraguay remained neutral under the leadership of Carlos Antonio Lopez from 1841 to 1862. While Lopez was cautious to not get his small country involved in larger wars, his son Francisco Solano Lopez idolized Napoleon and engaged in the extensive militarization of Paraguay, forming a Paraguayan Army modeled after Napoleon's Grand Armee. Lopez was eager to organize the continent's smaller nations in a political coalition designed to offset the power and influence of Brazil and Argentina, and, when civil war broke out in Uruguay in August 1864, Solano readily agreed to help the ruling Blancos crush the Brazilian and Argentine-backed Colorado rebellion.
On 13 November 1864, Paraguay started the war by capturing a Brazilain ship carrying the new governor of Mato Grosso up the river Paraguay. Paraguay began the war with 60,000 trained men, but they were badly equipped, wielding inaccurate muskets and carbines; they also lacked experienced officers, food, ammunition, and armaments. Paraguay launched an offensive into Brazilian Mato Grosso on 14 December 1864, followed by invasions of Rio Grande do Sul in early 1865 and Argentina's Corrientes Province at the same time. The Paraguayans made good progress in Mato Grosso and evacuated captured cities rather than give battle to the Brazilians, who withdrew from the area in April 1868 to focus on invading southern Paraguay. On 29 March 1865, Paraguay declared war on Argentina, but Solano failed to court the caudillo Justo Jose de Urquiza, who remained loyal to his country despite his rivalry with President Bartolome Mitre.
On 1 May 1865, Brazil, Argentina, and the newly empowered Colorado government in Uruguay formed a "Triple Alliance" against Paraguay, with Mirte as supreme commander of the allied forces. On 11 June 1865, the Brazilian Navy destroyed the Paraguayan Navy at the Battle of Riachuelo, thus cutting off Paraguay's access to the Rio de la Plata. Allied troops also defeated a Paraguayan army headed for Uruguay at the Battle of Yatay, forcing Solano to withdraw from Corrientes, even as his forces continued to have the upper hand in Rio Grande do Sul. By the end of 1865, the Triple Alliance's 42,000 infantry and 15,000 cavalry were on the offensive. In January 1866, the Paraguayans defeated the Allies at the Battle of Pehuajo, but they were unable to stop an Allied invasion in April 1866. The Paraguayans checked the Allied advance at the Battle of Estero Bellaco in May 1866, and he counterattacked with 25,000 troops against 35,000 Allied troops at the Battle of Tuyuti in May 1866. Lopez's army was narrowly defeated, and both armies suffered heavy losses.
The Paraguayans and Allies fought a series of seesaw battles, with the Paraguayans winning the Battle of Boqueron in July 1866 and the Brazilians winning the Battle of Curzuu in September. While Lopez showed a willingness to make peace with the Allies, he refused to step down from power, meaning a rejection of the Treaty of the Triple Alliance. The Allied armies were catastrophically defeated at the Battle of Curupayty in September 1866, ending their offensive for ten months, until July 1867. Meanwhile, Uruguayan president Venancio Flores was assassinated on fleeing to Uruguay, while Argentine-Brazilian relations deteriorated.
The Brazilians decided to create a unified command over Brazilian forces in Paraguay, entrusting it to Luis Alves de Lima e Silva in October 1866. The Triple Alliance besieged Humaita and narrowly fought off a Paraguayan attack at the Second Battle of Tuyuti in November 1867. By then, Paraguay had already lost 60,000 men, forcing Solano to conscript another 60,000 soldiers from slaves and children. Solano even executed his two brothers and two brothers-in-law for defeatism. By December 1867, nearly 46,000 Brazilians, 6,000 Argentines, and 500 Uruguayans were at the front. Humaita fell on 25 July 1868 after a long siege, but Lopez and the bulk of his army escaped. The Allies then advanced on the Paraguayan capital of Asuncion, but Solano left 12,000 troops to guard the Piquissiri River. In December 1868, the Brazilians launched the Dezembrada offensive against the Paraguayan rear. The Paraguayans inflicted heavy losses on the Brazilians at the Battle of Ytororo, but an entire Paraguayan division was destroyed at the Battle of Avay and the Paraguayans were again defeated at the Battle of Lomas Valentinas. On 24 December 1868, Lopez again refused to surrender, and Asuncion fell on 1 January 1869.
30,000 Allied soldiers looted almost every building in Asuncion over the next few months, while Solano Lopez was forced to go on the run. The Allies occupied Paraguay, while emerging domestic factions filled the power vacuum and helped form a provisional government favorable to the Argentines. Emperor Pedro II of Brazil's son-in-law Gaston, Count of Eu was given 21,000 troops to wage the Campaign of the Hills against the pro-Solano resistance, winning the Battle of Piribebuy and the Battle of Acosta Nu. Solano Lopez ordered his troops to kill any defeatist colleagues, and soldiers were forced to fight to the bitter end. On 1 March 1870, Solano Lopez was killed at the Battle of Cerro Cora while resisting capture.
Disease and war killed up to 60-70% of the total prewar population, including up to 90% of the male population. Paraguay also lost 54,000 square miles of territory to the allies, with Brazil acquiring Mato Grosso do Sul and Argentina capturing Misiones Province. Paraguay's growth was permanently stunted, and its war debt went unpaid until Brazilian president Getulio Vargas pardoned Paraguay in 1943. Brazilian forces occupied Paraguay until 1876, when Argentina recognized its independence. In Uruguay, the Colorados would retain power until 1958.