Francisco Solano López in the context of Juan Manuel de Rosas

The Paraguayan War (Spanish: Guerra del Paraguay, Portuguese: Guerra do Paraguai, Guarani: Paraguái Ñorairõ), also known as the War of the Triple Alliance (Spanish: Guerra de la Triple Alianza, Portuguese: Guerra da Tríplice Aliança, Guarani: Ñorairõ Triple Alianza Rehegua), was a South American war that lasted from 1864 to 1870. It was fought between Paraguay and the Triple Alliance of Argentina, the Empire of Brazil, and Uruguay. It was the deadliest and bloodiest inter-state war in Latin American history. Paraguay sustained large casualties, but even the approximate numbers are disputed. Paraguay was forced to cede disputed territory to Argentina and Brazil. The war began in late 1864, as a result of a conflict between Paraguay and Brazil caused by the Uruguayan War. Argentina and Uruguay entered the war against Paraguay in 1865, and it then became known as the "War of the Triple Alliance".

After Paraguay was defeated in conventional warfare, it conducted a drawn-out guerrilla resistance, a strategy that resulted in the further destruction of the Paraguayan military and the civilian population. Much of the civilian population died due to battle, hunger, and disease. The guerrilla war lasted for 14 months until president Francisco Solano López was killed in action by Brazilian forces in the Battle of Cerro Corá on 1 March 1870. Argentine and Brazilian troops occupied Paraguay until 1876.

The Uruguayan War (10 August 1864 – 20 February 1865) was fought between Uruguay's governing Blanco Party and an alliance consisting of the Empire of Brazil and the Uruguayan Colorado Party, covertly supported by Argentina. Since its independence, Uruguay had been ravaged by intermittent struggles between the Colorado and Blanco factions, each attempting to seize and maintain power in turn. The Colorado leader Venancio Flores launched the Liberating Crusade in 1863, an insurrection aimed at toppling Bernardo Berro, who presided over a Colorado–Blanco coalition (fusionist) government. Flores was aided by Argentina, whose president Bartolomé Mitre provided him with supplies, Argentine volunteers and river transport for troops.

The fusionism movement collapsed as the Colorados abandoned the coalition to join Flores' ranks. The Uruguayan Civil War quickly escalated, developing into a crisis of international scope that destabilized the entire region. Even before the Colorado rebellion, the Blancos within fusionism had sought an alliance with Paraguayan dictator Francisco Solano López. Berro's now purely Blanco government also received support from Argentine federalists, who opposed Mitre and his Unitarians. The situation deteriorated as the Empire of Brazil was drawn into the conflict. Almost one fifth of the Uruguayan population were considered Brazilian. Some joined Flores' rebellion, spurred by discontent with Blanco government policies that they regarded as harmful to their interests. Brazil eventually decided to intervene in the Uruguayan affair to reestablish the security of its southern frontiers and its regional ascendancy.

The Battle of Cerro Corá (Spanish pronunciation: [ˈsero koˈɾa]) was the last battle of the Paraguayan War, fought on 1 March 1870, in the vicinity of Cerro Corá, 454 kilometres (280 mi) northeast of Paraguay's capital Asunción. It is known for being the battle in which Francisco Solano López, Paraguayan president, was killed at the hands of the Imperial Brazilian Army.

The Paraguayan War was dragging on for more than five years and, after numerous battles, the Paraguayan army had been reduced to the elderly, the sick and children. The battle of Acosta Ñu was the last major combat of the war, which from then on was restricted to occasional skirmishes in the final months of 1869 and beginning of 1870. During this period, the Count of Eu, the allied commander-in-chief, organized expeditions in search of Solano López, following the path his column had taken. Along the way, López's and Eu's men made the civilian population suffer, either because of alleged conspiracies against López, or because of the looting and mistreatment inflicted by imperial troops. On 8 February 1870, López and his column reached Cerro Corá.