The Battle of San Lorenzo was fought on 3 February 1813 in San Lorenzo, Argentina, then part of the United Provinces of the Ro de la Plata. The royalist troops, were composed of militiamen recruited in Montevideo under the command of militia captain Antonio Zabala that was defeated by the Regiment of Mounted Grenadiers, under the command of Jos de San Martn. This battle was the baptism by fire for this military unit, and for San Martn in the Spanish American wars of independence.

Montevideo, a royalist stronghold during the Argentine War of Independence, was under siege by Jos Gervasio Artigas. Those in the city raided population centres along the nearby rivers for supplies. San Martn, who shortly before had arrived in Buenos Aires and formed the regiment, followed the royalist ships to San Lorenzo. The area around San Lorenzo formed a large empty plain, so the regiment hid inside the San Carlos Convent during the night and San Martn studied the battlefield and the enemy ships from the tower. The battle started at dawn, when the grenadiers made a surprise pincer movement to trap the enemy forces. One column was led by San Martn, and the other by Justo Germn Bermdez. San Martn fell from his horse, and was nearly killed, but Juan Bautista Cabral and Juan Bautista Baigorria intervened and saved him. The royalists were defeated, but continued to raid villages for some time afterwards.

This battle was the only one that San Martn fought in the modern territory of Argentina. The city of San Lorenzo keeps historic memorials of the battle and it is referenced in the San Lorenzo march.

José Francisco de San Martín y Matorras (25 February 1778 – 17 August 1850), known simply as José de San Martín (Spanish pronunciation: [xoˈse ðe san maɾˈtin] (listen)) or the Liberator of Argentina, Chile and Peru, was an Argentine general and the primary leader of the southern and central parts of South America's successful struggle for independence from the Spanish Empire who served as the Protector of Peru. Born in Yapeyú, Corrientes, in modern-day Argentina, he left the Viceroyalty of the Río de la Plata at the early age of seven to study in Málaga, Spain.

In 1808, after taking part in the Peninsular War against France, San Martín contacted in London South American supporters of independence from Spain. In 1812, he set sail for Buenos Aires and offered his services to the United Provinces of the Río de la Plata, present-day Argentina. After the Battle of San Lorenzo and time commanding the Army of the North during 1814, he organized a plan to defeat the Spanish forces that menaced the United Provinces from the north, using an alternative path to the Viceroyalty of Peru. This objective first involved the establishment of a new army, the Army of the Andes, in Cuyo Province, Argentina. From there, he led the Crossing of the Andes to Chile, and triumphed at the Battle of Chacabuco and the Battle of Maipú (1818), thus liberating Chile from royalist rule. Then he sailed to attack the Spanish stronghold of Lima, Peru.

On 12 July 1821, after seizing partial control of Lima, San Martín was appointed Protector of Peru, and Peruvian independence was officially declared on 28 July. On 26 July 1822, after a closed-door meeting with fellow libertador Simón Bolívar at Guayaquil, Ecuador, Bolívar took over the task of fully liberating Peru. San Martín unexpectedly left the country and resigned the command of his army, excluding himself from politics and the military, and moved to France in 1824. The details of that meeting would be a subject of debate by later historians.

San Martín is regarded as a national hero of Argentina, Chile and Peru, a great military commander, and one of the Liberators of Spanish South America. The Order of the Liberator General San Martín (Orden del Libertador General San Martín), created in his honor, is the highest decoration conferred by the Argentine government.