The trouble began in May 1810, when Buenos Aires deposed the Spanish viceroy and claimed the right to govern itself while Spain lay under Napoleon's heel. Across the river in Montevideo, royalist authorities refused to accept it and blockaded the new government's port. Montevideo had what Buenos Aires lacked: a real naval base and the Spanish Atlantic fleet. The revolutionary Junta Grande understood the danger at once. Whoever controlled the Rio de la Plata and the Parana controlled the whole region's trade and movement. So they resolved to build a fleet from nothing - and handed the job to a navy secretary named Francisco de Gurruchaga.

Gurruchaga bought five vessels from local owners and armed three of them: a schooner christened Invencible, a brigantine named 25 de Mayo for the revolution's birthday, and a sloop called America. The guns were largely cast-offs, withdrawn as obsolete. Crewing the ships was harder still. The people of the Pampas were horsemen, not sailors, so the officers hired foreigners who mostly could not speak Spanish and filled the lower ranks with locals. Command went to Juan Bautista Azopardo, a Maltese privateer who had once fought against the British invasions of 1806 and 1807. His second on the 25 de Mayo was Hipolito Bouchard - a French-born adventurer who, seven years later, would carry the Argentine flag around the world and briefly seize Monterey, California from Spain.

The fleet's first mission was a rescue. General Manuel Belgrano's army, sent to bring Paraguay into the revolution, had been beaten and needed reinforcements rushed upriver. Azopardo was ordered to carry troops and guns north. But Montevideo learned of the plan and sent the skilled officer Jacinto Romarate to intercept, with seven warships better armed than anything the rebels had. Near San Nicolas, Azopardo chose to fight rather than run. He landed some of his cannon on the bank and prepared his sailors and militia to fight from the shore - turning the riverbank into part of his line, the way defenders of the Parana would again decades later.

The battle went badly almost from the start. Two royalist brigantines grounded near shore and took fire, but Azopardo could not board them, and they worked free. The America was holed at the bow and abandoned as she flooded. Aboard the 25 de Mayo, the green crew panicked and leapt overboard despite Bouchard's efforts to hold them. That left the Invencible fighting alone. Her crew held out for nearly two hours until the royalists boarded. Azopardo moved to blow up the magazine and take the enemy with him, but his own wounded begged him not to, and he surrendered. He was shipped as a prisoner to Ceuta in Spain, where he later faced a death sentence for his revolutionary loyalties before it was commuted. A Buenos Aires inquiry faulted his inexperience yet expressly recognized his bravery. Freed in 1820 when a liberal uprising in Spain opened the prisons, Azopardo returned to South America, and the following year Buenos Aires restored him to the navy he had helped to found. The defeat at San Nicolas had cost the revolution its fleet - and control of the rivers passed to Montevideo's royalists until William Brown rebuilt Argentine sea power in 1813 - but the men forged in that losing fight became the foundation of a naval tradition that endures.