The War of the Triple Alliance, also known as the Paraguayan War, and the Great War in Paraguay itself, is fought from 1864 to 1870, and causes more deaths than any other South American war.

It is fought between Paraguay and the allied countries of Argentina, Brazil, and Uruguay.

The start of the war has been widely attributed to causes as varied as the aftereffects of colonialism in Latin America, the struggle for physical power over the strategic Río de la Plata region, Brazilian and Argentinian meddling in internal Uruguayan politics, British economic interests in the region, and the expansionist ambitions of Paraguayan president Francisco Solano López.

The outcome of the war is the utter defeat of Paraguay.

After the Triple Alliance defeats Paraguay in conventional warfare, the conflict turns into a drawn-out guerrilla-style resistance that devastates the Paraguayan population, both military and civilian.

One estimate places total Paraguayan losses — through both war and disease — as high as 1.2 million people, or 90% of its prewar population.

A different estimate places Paraguayan deaths at approximately 300,000 people out of its 500,000 to 525,000 prewar inhabitants.It will take decades for Paraguay to recover from the chaos and demographic imbalance in which it had been placed: what had been in name one of the first South American republics, Paraguay will not choose its first democratically elected president until 1993.In Brazil, the war helps bring about the end of slavery, moves the military into a key role in the public sphere, and causes a ruinous increase of public debt, which will take decades to pay, seriously reducing the country's growth.It has been argued that the war played a key role in the consolidation of Argentina as a nation-state.

After the war, that country becomes Latin America's wealthiest nation, a position that still holds in terms of human development.

For Uruguay, it is the last time that Brazil and Argentina will take such an interventionist role in its internal politics.