The Battle of Boquerón was fought on 16 July 1866 and the Battle of Sauce on 18 July 1866, between an allied force of Uruguayans, Brazilians, and Argentines on one side and Paraguay on the other in the Paraguayan War. The Spanish-born Uruguayan officer León de Pallejas (1816–1866) and the Paraguayan officer Elizardo Aquino were killed in the battle.

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."