The road to Sarandí began six months earlier, on a beach. In April 1825, thirty-three exiles led by Lavalleja landed at Playa de la Agraciada carrying a tricolor flag - blue, white, and red, stitched with the words Libertad o Muerte, liberty or death. Their land, the Banda Oriental, had been swallowed by Brazil and renamed Cisplatina, and these Thirty-Three Orientals meant to take it back. It seemed impossible. Yet through that spring and summer they besieged Montevideo, harried Colonia del Sacramento, and at the Battle of Rincón in September captured some 6,000 Brazilian horses - a blow that crippled the empire's cavalry.

Stung by Rincón, the Brazilians sent two columns of about a thousand men each to converge and destroy the rebels. Lavalleja tried to keep them apart and failed, so instead he gathered a force of matching size and chose his ground at the tips of the Sarandí creek, in what is now Uruguay's Florida Department. He drew his battle line along the high ground. Fructuoso Rivera held the left, anchored on the creek; Pablo Zufriategui stood in the center; Manuel Oribe took the right. When the imperial troops crossed the stream and climbed, they expected to find Rivera alone. Instead the whole Oriental army stood waiting on the ridge.

The Brazilian commander, Bento Manuel Ribeiro, saw the danger in the Oriental position and tried to wheel his forces rather than charge straight in. The maneuver forced Lavalleja to swing his own line from facing south to facing west, and the hurried pivot threw his ranks into confusion. At eight o'clock the Oriental guns opened fire and the battle broke loose. For a moment it nearly went wrong - the disciplined imperial troops of Colonel Joaquim Alencastre punched clean through the Oriental center and reached the reserves. Then Lavalleja committed those reserves himself, splitting the Brazilian formation in two. Rivera's horsemen flanked Alencastre; the cavalry surged; and the imperial army dissolved into a rout it could not stop.

Sarandí was the military answer to a political declaration that had already been made. Six weeks earlier, on 25 August 1825, the Congress of Florida had voted to rejoin the United Provinces of the River Plate, and the assembly in Buenos Aires had accepted them - a declaration that technically dragged Brazil and Argentina into open war. What remained was proving that war could be won. Sarandí did exactly that. The fighting was not finished. The Brazilians still held the northeast, and the Orientals had to storm the Fortress of Santa Teresa near the coast that December to loosen their grip. But the tide had turned on that ridge above the creek. The struggle that began with thirty-three men and a homemade flag had become a war between empires, and Uruguay was being born inside it.