At Mendoza, on January 17, San Martin reviewed his little army of 5,000, all Gaucho horsemen, as lightly clad and provisioned as the Indians of the Pampas. The women of Mendoza presented the force with a flag bearing the emblem of the Sun. San Martin held the banner aloft, declaring it "the first flag of independence which had been blest in South America." This same flag was carried through all the wars along the Pacific Coast. And under its tattered shreds San Martin was finally laid to rest sixty years later.
Marching from Mendoza, San Martin made a feint of crossing the Andes by way of Planchon, thereby inducing a Spanish column under Captain-General Marco del Ponte to concentrate at Talca. During the progress of these movements, San Martin and his followers crossed the mountains by the steep route of Putaendo and Cuevas. Three hundred miles of the stiffest mountain riding were covered in less than a fortnight. Early in February, San Martin's army, now barely 4,000 strong, descended upon Villa Nueva. On February 7, they fought their first battle on Chilean soil with the Spanish outposts at Chacabuco. Driving the Spaniards before him, San Martin, advanced into the plain, and presently joined forces with O'Higgins' infantry. New mounts were provided for the cavalry. At the strong post of Acuoncagua the Spaniards made a stand, but they were outnumbered by the insurgents. San Martin delivered a frontal attack, while O'Higgins outflanked the enemy with an impetuous charge, with the result that the whole Spanish force was routed beyond recovery. The officers fled to Valparaiso. By the middle of February, San Martin entered Santiago de Chile. A new republican junta was formed and complete independence of Spain was declared. O'Higgins assumed the position of dictator.
All Chile was free now except in the south. General Ordoñez, commanding the Spanish forces there, was defeated and fell back to Talcahuano. San Martin prepared to invade Peru. Anticipating such an attack, Abascal, the Spanish Viceroy of Peru, despatched Osorio with an expedition of 3,500 veterans, who had just arrived from Spain, to Talcahuano. As soon as these reinforcements came, Ordoñez set out from Talcahuano with the vanguard to march on Santiago de Chile, and met the patriot forces near Talca. The revolutionists largely outnumbered the Spaniards, but were poorly disciplined and ill-provisioned. While they lost time the Spanish main column under Osorio came up. Ordoñez took advantage of the clumsy manoeuvres of the revolutionists to drive a sharp attack between their two wings, piercing their centre. The battle was won after the first fifteen minutes. O'Higgins was wounded and had to be carried out of the fight. San Martin, with his right wing, fell back on San Fernando. With great difficulty O'Higgins managed to reach Santiago, where he was presently joined by San Martin. Steadily the Spanish column advanced on Santiago. The two revolutionary leaders by almost superhuman efforts succeeded in rallying and equipping a force of 5,000 defenders. On April 5, the Spanish army appeared before Santiago de Chile. Near the Maypo, nine miles from Santiago, the revolutionists took up a strong position. Osorio opened the battle about noon with artillery. Soon all the troops were engaged, the fiercest fight raging around a hacienda where San Martin and O'Higgins had their headquarters. Several times the ranch was lost and retaken. By sundown the Spaniards advanced all along the line. The battle seemed lost to the patriots. At this juncture, as the famous regiment of Burgos on the Spanish right was drawing in its deployed lines for a final column attack, Colonel O'Brien, at the head of the insurgent cavalry reserves, charged into the opening and overthrew the Burgos battalions. O'Higgins immediately charged the rest of the Spanish right wing, and San Martin simultaneously attacked in the centre. The whole Spanish army gave away. More than 2,000 Spaniards were killed and wounded. Osorio with his staff escaped to Peru. The victory of Santiago not only freed Chile, but left Peru open to the revolutionists.