War of the Triple Alliance
The War of the Triple Alliance, also known as the Paraguayan War or the Great War, pitted Paraguay against the triple alliance of Argentina, Brazil, and Uruguay. Lasting from 1864 until 1870, it is the bloodiest war in the history of South America. The war drew to a close with the death Francisco Solano López, Paraguay's second president, at the hands of a Brazilian lancer.
The pretext of the war started with López's August 30, 1864, ultimatum to Brazil after they entered Uruguay. After ignoring López's warnings, Paraguay seized a Brazilian merchant ship sailing through Paraguayan waters to reach the Brazilian state of Mato Grosso. Paraguay asked to use Argentina's strategic Corrientes corridor to launch an attack on Brazilian forces (Argentina already gave Brazil equivalent access to its waterways). Citing neutrality, however, Argentina declined access for Paraguay. López responded with a declaration of war on Argentina on March 18, 1865.
On May 1, 1865, the Triple Alliance was formally aligned.