At the end of 1825, the newly independent Empire of Brazil went to war with the newly independent Republic of Argentina – then called the United Provinces of the River Plate – over control of the north bank of the Plate Estuary and consequently of the river itself. It was a replay of the long-standing dispute between the Portuguese and Spanish colonial Empires over what was confusingly called the ‘Banda Oriental’ (properly the ‘Banda Oriental del Uruguay’). Originally a Spanish province, the area had been occupied by Brazil in the confusion which followed the Napoleonic Wars. In 1825, a rebellion against Brazilian rule brought the United Provinces into the conflict and drove the two countries to war. It lasted for two years and resulted in a stalemate – the United Provinces dominating on land, the Brazilians at sea. Eventually, the state of Uruguay was created as a compromise and a buffer between the two.

The two nations that confronted each other in the River Plate were strikingly different. The United Provinces was a republic, with a small, largely immigrant population scattered over a vast land mass politically dominated by the city and province of Buenos Aires. It had a large international trade, carried out principally in British, American and French vessels, but few maritime pretensions. Its naval forces were modest and, at the beginning of the war, consisted of no more than six small warships and a dozen gunboats for port defence. Brazil, by contrast, was a monarchy, ruled by the former Crown Prince of Portugal who, as the Emperor Pedro I, had led the country to independence in 1822–23. It was a vast country with a huge coastline, an extensive seaborne commerce and a large and experienced navy comprising some 96 ships of all sizes carrying 690 guns. The strategy of the two nations in the maritime war reflected their circumstances: Brazil's aim was to strangle the trade of Buenos Aires by blockade; while that of the United Provinces was to keep the waters of the Plate open while unleashing a swarm of privateers on Brazil's sea borne commerce.

Manpower and Foreign Recruitment

A remarkable feature of the war is that the navies of both sides in the conflict relied heavily on foreign sailors – notably British and Americans – to command and man their ships. This tradition had been established during the struggles for independence from Spain and Portugal.