Argentine War of Independence Chile, allied with the Mapuches. On April 4, Argentine Colonel Juan Gregorio de Las Heras had occupied Concepción, but the Royalists retreated to Talcahuano. In early 1818, Royalist reinforcements from the Viceroyalty of Peru arrived, commanded by general Mariano Osorio, and advanced towards the capital. San Martín then turned to scorched earth tactics and ordered the evacuation of Concepción, which he thought was impossible to defend. On 18 February 1818, the first anniversary of the battle of Chacabuco, Chile declared its independence from the Spanish Crown. On March 18, 1818, Osorio led a surprise attack on the joint Argentine-Chilean army,
José de San Martín that it was not possible to defend Concepción, so he ordered O'Higgins to leave the city. 50,000 Chileans took cattle and grain and moved north, burning everything else, so that they did not leave supplies for the royalists. As he had done with the Tucumán Congress, San Martín urged a declaration of independence, to legitimize the government and the military actions. The Chilean Declaration of Independence was issued on 18 February 1818, one year after the battle of Chacabuco. San Martín, Las Heras and Balcarce met in Curicó, and the royalists in Talca, in a plain known as "Cancha rayada".