Battle of San Lorenzo - Prelude

Prelude

Although Buenos Aires had suffered a difficult period in its war for independence, its prospects were improving by 1812. Even though the defeats of Manuel Belgrano during the Paraguay campaign and Juan José Castelli in the first Upper Peru campaign had generated a political crisis, Belgrano's victory at Tucuman had given new fresh hope to the revolution, which would be strengthened shortly afterwards with the victory at the Battle of Salta. Montevideo, capital of the Viceroyalty of the Río de la Plata since the May Revolution, was under siege by both an army from Buenos Aires led by José Rondeau and an Uruguayan one led by José Gervasio Artigas. The city, however, maintained its naval supremacy over Buenos Aires and their ships raided the coasts of the Paraná and Uruguay rivers to gather supplies, despite the siege. Montevideo organized a navy to destroy the gun batteries at Rosario and Diamante, two population centers along the Parana, but were prevented from doing so as Buenos Aires dismantled them knowing that they could not defend them.

The Royalist expeditionary navy that would raid San Lorenzo comprised eleven ships of varying sizes, two hundred and fifty soldiers and fifty sailors. They moved into the Paraná through its tributary, the Paraná Guazú River, slowed down by headwinds. The Second Triumvirate promoted José de San Martín to colonel and instructed him to follow them with the Regiment of Mounted Grenadiers and stop the raids. San Martín was influenced by Napoleonic warfare and trained the regiment with the most recent military techniques used in the Napoleonic Wars.

San Martín moved the regiment from Retiro to Rosario, nearing the river at San Pedro and San Nicolás. He was following the Spanish ships and moved at night to avoid detection. San Martín had one hundred and twenty men for this action, reinforced at Rosario by a militia of seventy men under the command of Celedonio Escalada. Those reinforcements included twenty two rifleman, thirty cavalry, a small cannon and men armed with knives. Escalada had made other actions against the royalists before this battle. San Martín discovered that the royalists intended to pillage the San Carlos Convent and pressed the march to arrive there first. A hundred royalists landed on San Lorenzo, but the only food available to them was some chickens and watermelons. Aware of the risk of pillage, the population had removed the cattle from the area before the royalists arrived. Escalada arrived in San Lorenzo before the bulk of the patriot army, but the dust trail from the path to Rosario revealed their presence. Escalada attacked them but their ship had a longer range than his cannon, keeping him at bay. He was forced to retire when he found a Paraguayan prisoner who had escaped from the ship. The Paraguayan disclosed the size of the royalist army and their plan of attacking the convent with a larger force, suspecting that the local money was kept in it. They did not attack the convent right away, requiring time to prepare the two field cannons. Escalada returned with San Martin and relayed the news. The march from Retiro to the convent took only five days, thanks to the cadet Ángel Pacheco. Pacheco moved ahead of the regiment and prepared horses in advance at the relay positions. The whole army arrived on scene during the night of 2 February and hid inside the convent. They entered through the rear door and were not allowed to light fires or speak during the night. San Martín studied the enemy and the battlefield from the convent's tower, using a monocular.