The Paraguayan War, also known as the War of the Triple Alliance, was a South American war that lasted from 1864 to 1870. This war 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 lost their lives 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.