The Uruguayan War (10 August 1864 – 20 February 1865) was fought between Uruguay's governing Blanco Party and an alliance consisting of the Empire of Brazil and the Uruguayan Colorado Party, covertly supported by Argentina. Since its independence, Uruguay had been ravaged by intermittent struggles between the Colorado and Blanco factions, each attempting to seize and maintain power in turn. The Colorado leader Venancio Flores launched the Liberating Crusade in 1863, an insurrection aimed at toppling Bernardo Berro, who presided over a Colorado–Blanco coalition (fusionist) government. Flores was aided by Argentina, whose president Bartolomé Mitre provided him with supplies, Argentine volunteers and river transport for troops.
The fusionism movement collapsed as the Colorados abandoned the coalition to join Flores' ranks. The Uruguayan Civil War quickly escalated, developing into a crisis of international scope that destabilized the entire region. Even before the Colorado rebellion, the Blancos within fusionism had sought an alliance with Paraguayan dictator Francisco Solano López. Berro's now purely Blanco government also received support from Argentine federalists, who opposed Mitre and his Unitarians. The situation deteriorated as the Empire of Brazil was drawn into the conflict. Almost one fifth of the Uruguayan population were considered Brazilian. Some joined Flores' rebellion, spurred by discontent with Blanco government policies that they regarded as harmful to their interests. Brazil eventually decided to intervene in the Uruguayan affair to reestablish the security of its southern frontiers and its regional ascendancy.