sentences:  Viceroy Joaquín de la Pezuela ordered De la Serna to attack Argentine insurgents in the province of Tucumán, but De la Serna opposed this plan, citing insufficient forces.De la Serna had advanced as far as Salta when the Spanish were surprised by the appearance in February 1817 of José de San Martín's Army of the Andes in Chile.San Martín had made an arduous, 21-day crossing of the mountains from Argentina.