The Paraguayan War, also known as the War of the Triple Alliance, 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 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.
Since their independence from Portugal and Spain in the early 19th century, the Empire of Brazil and the Spanish-American countries of South America were troubled by territorial disputes. Each nation in this region had boundary conflicts with multiple neighbors. Most had overlapping claims to the same territories, due to unresolved questions which stemmed from their former metropoles. Signed by Portugal and Spain in 1494, the Treaty of Tordesillas proved ineffective in the following centuries, as both colonial powers expanded their frontiers in South America and elsewhere. The outdated boundary lines did not represent the actual occupation of lands by the Portuguese and Spanish.
By the early 1700s, the Treaty of Tordesillas was deemed not useful, and it was clear to both parties that a newer treaty had to be drawn based on feasible boundaries.