The Cisplatine War was an armed conflict fought in the 1820s between the Empire of Brazil and the United Provinces of the Río de la Plata over control of Brazil's Cisplatina province. It was fought in the aftermath of the United Provinces' and Brazil's independence from Spain and Portugal, respectively, and resulted in the independence of Cisplatina as the Oriental Republic of Uruguay.

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.