Paraguay declares war on Brazil, 13 December 1864

Background. In 1863, the Empire of Brazil, which did not have friendly relations with Paraguay, began providing military and political support to an incipient rebellion in Uruguay led by Venancio Flores and his Colorado Party against the fusionist government of Bernardo Berro and his successor Atanasio Aguirre, both of the Blanco Party. In April 1864, one year after the beginning of the Cruzada Libertadora, Brazilian minister José Antônio Saraiva arrived in Uruguayan waters with the Imperial Fleet to demand payment for damages caused to Rio Grande do Sul farmers in border conflicts with Uruguayan farmers.

Diplomatic Escalation. Tensions peaked in late 1864. On 4 August, Brazil sent an ultimatum to Uruguay. Paraguay responded on 2 September, warning that if Brazil proceeded with its measures, it would be forced to make its protest effective. This diplomatic standoff set the stage for formal hostilities, as Paraguay sought to defend the Uruguayan National Party against Brazilian intervention.

Declaration and Mobilization. In December 1864, López formally declared war on Brazil. This declaration was followed by the dispatch of forces to invade Mato Grosso. The conflict initially involved Paraguay and Brazil, but later expanded when Argentina and Uruguay joined the war against Paraguay in 1865, leading to the formation of the Triple Alliance.