Cochrane was already famous when Brazil came calling. He had ended Spanish rule in Chile three years earlier by capturing Valdivia with a handful of ships and an audacious mind, and his name alone could rattle a Portuguese captain. Independent Brazil, declared in September 1822, needed exactly that kind of reputation. The young empire had ports, pride, and a coastline to defend, but almost no trained sailors of its own - Portugal had spent three centuries making sure of that. In December 1822, emissaries sailed to London with an offer: command our fleet, keep a share of every Portuguese prize you take. Cochrane said yes. He arrived in Rio on 13 March 1823 with a handful of officers who had served with him in the Pacific, negotiated his pay, and took command of the Pedro I eight days later. Within a month he was sailing for Bahia with orders to blockade Salvador and break Portuguese resistance at sea.

The Pedro I was rated a 74-gun ship of the line, though Cochrane thought her closer to a 64 by Royal Navy standards. What he found when he inspected her was worse than the rating. The sails tore in moderate wind. The gunpowder bags were so rotten that every shot risked setting off the cannon's next charge, forcing gun crews to swab the bores with sponges between firings. His own sailors cut up spare flags to sew new powder cartridges. The flintlocks every modern warship carried were absent; crews would have to light their guns the old way, with slow match and prayer. His complement was a mosaic of loyalties: 160 English and North American veterans, 130 Black marines newly freed from slavery, and a thin layer of Portuguese-born sailors paid less than half the going wage. The marines, remembering too well what unpaid labor meant, refused to scrub decks. The Portuguese sailors scrubbed instead of drilling. Cochrane estimated he was 120 men short, and that was before anyone had fired a shot.

Shortly after sunrise on 4 May 1823, Cochrane's lookouts counted thirteen Portuguese sails to leeward under Chief of Division João Félix Pereira de Campos. The odds were worse than the numbers suggested - the Portuguese flagship Dom João VI alone carried 88 guns. Cochrane's plan was classic Nelsonian boldness: cut the enemy line astern of the frigate Constituição, rake the four rearmost ships before the van could wear round, and create a local advantage where global inferiority made no sense. At noon the Pedro I opened fire on the troopship Princesa Real. Then the morning's arithmetic collapsed. The Piranga, Nichteroy, and Liberal - their Portuguese-born crews unwilling to shoot at countrymen - never closed the range. Two sailors in the Pedro I's magazine locked the powder boys inside rather than feed the British guns. Only the Maria da Glória, crewed by Brazilians trained by the French captain Beaurepaire, kept firing. Cochrane saw what was happening, disengaged before disaster, and somewhere across the water the crew of the Brazilian schooner Real tried to surrender their own ship to the Portuguese. Cochrane stopped them.

A tactical defeat became a strategic pivot. Cochrane retired to Morro de São Paulo, an island refuge fifty miles south, and did what he had done in the Pacific: he improvised. The best Brazilian sailors from the other ships were transferred to the Pedro I and the Maria da Glória. Captain Pio and men of unquestioned loyalty took the unreliable vessels. With just two warships Cochrane strangled the Portuguese supply line into Salvador. By early July the occupying force under Brigadier Inácio Luís Madeira de Melo was starving. On the morning of 2 July 1823, Madeira evacuated Salvador and steered for Lisbon. Cochrane chased him all the way to the Portuguese coast, capturing seven transports during the pursuit. Bahian troops marched into Salvador the same day; Brazil celebrates 2 July as the true end of the war of independence. The 130 formerly enslaved marines who had stood by their guns that noon off Salvador had helped win something their own country had not yet decided how to give them.