Like Miranda, he had been an officer in the Spanish army and had returned to his native land an ardent apostle of independence.
Quick to realize the fact that, so long as Chile remained under royalist control, the possibility of an attack from that quarter was a constant menace to the safety of the newly constituted republic, he conceived the bold plan of organizing near the western frontier an army--composed partly of Chilean refugees and partly of his own countrymen--with which he proposed to cross the Andes and meet the enemy on his own ground.
Among these fugitives was the able and valiant Bernardo O'Higgins, son of an Irish officer who had been viceroy of Peru.

Cooperating with O'Higgins, San Martin fixed his headquarters at Mendoza and began to gather and train the four thousand men whom he judged needful for the enterprise.
By January, 1817, the "Army of the Andes" was ready.

To cross the mountains meant to transport men, horses, artillery, and stores to an altitude of thirteen thousand feet, where the Uspallata Pass afforded an outlet to Chilean soil.

This pass was nearly a mile higher than the Great St.Bernard in the Alps, the crossing of which gave Napoleon Bonaparte such renown.

On the 12th of February the hosts of San Martin hurled themselves upon the royalists entrenched on the slopes of Chacabuco and routed them utterly.