Background and development of events
In 1810, the May Revolution had forced the Spanish to abandon Buenos Aires, but they held on to the Banda Oriental (present-day Uruguay), as Spain moved the headquarters of the Viceroyalty of the Río de la Plata to Montevideo. At the beginning of April 1811, the revolutionary José Gervasio Artigas returned to the Banda Oriental with approximately 180 men provided by the Government of Buenos Aires. On April 11, he issued the Mercedes Proclamation, assuming control of the revolution.
The Governor of Montevideo and new Viceroy of Río de la Plata, Francisco Javier de Elío, appointed frigate-captain
José Posadas at the head of the forces loyal to Spain. Posadas installed his headquarters at San Isidro Labrador de Las Piedras near Montevideo, to provoke a decisive battle against the revolutionaries.
Meanwhile, José Artigas was camped near Nuestra Señora de Guadalupe with an army of a thousand men. The army of Posadas counted 1230 men, of which some 200 would defect to Artigas in the midst of battle.
The battle was fought on May 18 at Las Piedras and resulted in a total victory for the revolutionaries. José Posadas capitulated. It was at this occasion that Artigas pronounced his famous sentence "Curad a los heridos, clemencia para los vencidos" (Cure the injured, mercy to the vanquished), an unusual decision in those times, referring to the Spanish wounded and prisoners. One of the casualties on the revolutionary side was Manuel Artigas, nephew of José Artigas.
Both armies fought in the name of King Ferdinand VII of Spain.