The Uruguayan War 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 Empire of Brazil was a 19th-century state that broadly comprised the territories which form modern Brazil and Uruguay until the latter achieved independence in 1828. Its government was a representative parliamentary constitutional monarchy under the rule of Emperors Pedro I and his son Pedro II. A colony of the Kingdom of Portugal, Brazil became the seat of the Portuguese Empire in 1808, when the Portuguese Prince regent, later King Dom John VI, fled from Napoleon's invasion of Portugal and established himself and his government in the Brazilian city of Rio de Janeiro. John VI later returned to Portugal, leaving his eldest son and heir-apparent, Pedro, to rule the Kingdom of Brazil as regent. On 7 September 1822, Pedro declared the independence of Brazil and, after waging a successful war against his father's kingdom, was acclaimed on 12 October as Pedro I, the first Emperor of Brazil. The new country was huge, sparsely populated, and ethnically diverse.