As in the United States and Mexico, the process of liberal nation-state consolidation in South America took place through military conflict. The War of the Triple Alliance, or Paraguay War (1865–1870), in which Paraguay fought Brazil, Argentina, and Uruguay, played a similar role to Mexico’s Wars of Reform and the U.S. Civil War in consolidating liberalism in South America. In 1865 Paraguayan leader Francisco Solano López declared war against the three neighboring countries after political competition between Argentina and Brazil threatened Paraguay’s use of Uruguay’s Atlantic port in Montevideo. Landlocked Paraguay depended on Montevideo as a shipping point for its imports and exports. Paraguay fought a five-year war against much larger neighbors until it was defeated in 1870. Paraguay’s perseverance can be ascribed to the intensive training of Solano López’s army, as well as the relative inability of the governments of Argentina and Brazil to mobilize armies and resources.

The war was devastating for Paraguay, which lost more than half its national population, including most adult men. But victory, too, was traumatic for Argentina and Brazil, where soul-searching took place about the conduct of the war: Argentines and Brazilians asked themselves why it had taken five years to defeat a much smaller neighbor. The war prompted debates about the need for economic modernization and the reform of national governments.

In Brazil, where Emperor Pedro II’s calls for volunteers to enlist in the army fell on deaf ears, the army enlisted slaves who, if they served honorably and survived, would be granted freedom. What did it mean when the free citizens of a nation would not mobilize to defend it, and when a nation prevailed only through the sacrifices borne by its slaves? For many, especially military officers who were veterans of the conflict, the lesson was that being a monarchy that relied on slavery made Brazil a backward nation. Veteran officers and liberal opponents of the war formed a movement to create a liberal republic and abolish slavery. These republicans overthrew monarchy in 1889 and installed a liberal regime that lasted until 1930.

In Argentina, after the war with Paraguay, a succession of liberal leaders beginning with Domingo Sarmiento (pres. 1868–1874) also pressed modernizing reforms intended to establish the political authority of the capital, Buenos Aires, over the rest of Argentina; take possession of frontier regions; institute universal public education; and strengthen economic production. Economic measures in Argentina were the most far-reaching and drew on the experience of the United States in settling its western frontier as a model. In a military campaign called the Conquest of the Desert (1878–1885), Argentine troops seasoned by war with Paraguay took control of the lands of Mapuche Indians in the southern region of Patagonia and opened new lands for sale to ranchers. The wars were accompanied by ambitious railroad construction that linked inland areas to the coast to transport goods, the introduction of barbed wire fencing that intensified ranching capabilities, and the development of new strains of cattle and wheat that increased production. The Conquest of the Desert and the government’s land distribution policies transformed Argentina’s countryside into highly productive ranch lands and farmlands whose exports competed directly with the ranches of the North American West.