Montevideo, a royalist stronghold during the Argentine War of Independence, was under siege by José Gervasio Artigas. Those in the city raided population centres along the nearby rivers for supplies. San Martín, 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 Martín 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 Martín, and the other by Justo Germán Bermúdez. San Martín 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.