The Cisplatine War (), also known as the Argentine-Brazilian War () or, in Argentine and Uruguayan historiography, as the Brazil War (Guerra del Brasil), the War against the Empire of Brazil (Guerra contra el Imperio del Brasil) or the Liberating Crusade (Cruzada Libertadora) in Uruguay, was an armed conflict in the 1820s between the United Provinces of the Río de la Plata and the Empire of Brazil over Brazil's Cisplatina province, in the aftermath of the United Provinces' and Brazil's independence from Spain and Portugal respectively. It resulted in the independence of Cisplatina as the Oriental Republic of Uruguay.
Background
Led by José Gervasio Artigas, the region known as the Banda Oriental, in the Río de la Plata Basin, revolted against Spanish rule in 1811, against the backdrop of the 1810 May Revolution in Buenos Aires as well as the regional rebellions that followed in response to Buenos Aires' pretense of primacy over other regions in the Viceroyalty of the Río de la Plata. In the same context, the Portuguese Empire, then headquartered in Rio de Janeiro, took measures to solidify its hold on Rio Grande do Sul and to annex the region of the former Eastern Jesuit Missions.
From 1814 on, the Provincia Oriental, led by Artigas, joined forces with the provinces of Santa Fe and Entre Rios in a loose confederation called the Federal League, which resisted Buenos Aires' authority. After a series of banditry incidents in the territory which was previously claimed by the Portuguese Empire, in what is today the state of Rio Grande do Sul in Brazil, the now United Kingdom of Portugal, Brazil and the Algarves invaded the Banda Oriental in 1816.
Artigas was finally defeated by the Luso-Brazilian troops in 1820 at the Battle of Tacuarembó. The United Kingdom of Portugal, Brazil and the Algarves then formally annexed the Banda Oriental as a province of the Kingdom of Brazil, under the name Cisplatina, with support from local elites. With the annexation, the United Kingdom of Portugal, Brazil and the Algarves now enjoyed strategic access to the Río de la Plata and control of the estuary's main port, the city of Montevideo.
After Brazilian independence, in 1822, Cisplatina remained as a province of the newly formed Empire of Brazil. It sent two delegates to the 1823 Constituent Assembly that was tasked with drafting Brazil's first constitution. The constitution drafted by the Assembly was rejected by Emperor Pedro I, who dissolved the Assembly and issued a constitution himself in 1824. Under the 1824 Constitution, the Cisplatina province enjoyed a considerable degree of autonomy, more so than other provinces within the Empire.
Conflict
While initially supporting the United Kingdom of Portugal, Brazil and the Algarves' intervention in the Banda Oriental, the United Provinces of the Río de la Plata later urged the local populace to rise up against Brazilian authority, covertly giving them political and material support aiming to establish sovereignty over the region.
Rebels led by Fructuoso Rivera and Juan Antonio Lavalleja carried on resistance against Brazilian rule. On 25 August 1825, an assembly of delegates from all over the Banda Oriental met in Florida and declared its independence from Brazil, while also declaring, at the same time, its allegiance to the United Provinces of the Río de la Plata. On 25 October 1825 the congress of the United Provinces declared the annexation of the Banda Oriental. In response, Brazil declared war on the United Provinces on 10 December 1825.
The two navies which confronted each other in the Río de la Plata and the South Atlantic were in many ways opposites. The Empire of Brazil was a major naval power with 96 warships, large and small, an extensive coastal trade and a large international trade carried on mostly in British, French and American ships. The United Provinces had similar international trading links but had few naval pretensions. Its navy consisted of only half a dozen warships and a few gunboats for port defence. Both navies were short of indigenous sailors and relied heavily on British—and, to a lesser extent—American and French officers and sailors, the most notable of which were the Irish born admiral William Brown, and the commander of the Brazilian inshore squadron, the English commodore James Norton.