On January 17, 1817, he set out with this force and began the crossing of the Andes. Careful planning on his part had meant that the royalist forces in Chile were deployed to meet threats that did not exist, and his crossing went unopposed. Nonetheless, the Army of the Andes (as San Martin's force was called) suffered heavy losses during the crossing, losing as much as one-third of its men and more than half of its horses. San Martin found himself allying with Chilean patriot Bernardo O'Higgins, who commanded his own army.