Argentine Republic, the Chileans were not idle. They saw that the only way of insuring their independence was by the creation of a navy. Through its agent in London, the Chilean government secured the services of Lord Cochrane, an English naval officer of great distinction and remarkable talents, who by a curious turn of fortune had been brought into unmerited disgrace and dismissed from the British service.[31] He reached Valparaiso in November and hoisted his flag on board the O'Higgins, December 22, 1818. During the course of the next year, Cochrane made two attempts to take Callao, the seaport of Peru, but without success beyond harassing the enemy in some of the smaller coast towns. In February, 1820, by a brilliant move, he captured Valdivia, a strongly fortified town still held by the Spaniards in southern Chile.

San Martin returned to Chile in January, 1820, and began to assemble at Valparaiso the army destined for the invasion of Peru. Of the 5,000 men, two-thirds were from Buenos Aires, while nearly all of the officers were Argentine or European volunteers. Of 65 foreign officers, 37 were British and 3 were from the United States. There were, besides, 30 English officers in the Chilean navy. The expedition sailed on August 21, 1820, on board the fleet commanded by Cochrane. San Martin landed his army at Pisco, to the south of Lima, and sent an expedition into the interior under General Arenales, who had served the Patriots for years in Upper Peru. In October, San Martin reëmbarked his troops and landed them again at Huacho, a point seventy miles north of Lima. Meanwhile the Spanish squadron, completely demoralized by the appearance on the Pacific of Lord Cochrane, whose daring exploits were well known, was lying under the guns of Callao Castle. On the night of November 5, 1820, Lord Cochrane led a force of volunteers, consisting of 180 seamen and 100 marines, in open boats right under the batteries of Callao, surprised and overpowered the crew of the Esmeralda, the largest Spanish frigate, and, cutting her loose, carried her off to his own squadron.

After Cochrane's exploit at Callao, the moral effect of which was very great, he urged upon San Martin an immediate advance upon Lima, but San Martin had two campaigns before him, the one military, the other political. On first landing at Pisco he had issued an order to his army in which he said, "Remember that you are come not to conquer, but to liberate a people; the Peruvians are our brothers." And in spite of the impatience and restlessness of his officers, he steadily adhered to his plan, to the no small loss of his military prestige and ultimately to his retirement from the scene of activity. His purpose was by the presence of the liberating army to give the people of Peru a chance to rise and throw off the yoke of Spain. To this end he scattered proclamations and addresses of a revolutionary character broadcast through the land and quietly awaited results. The contest in Peru, he said, was not a war of conquest and glory, but entirely of opinion; it was a war of new and liberal principles against prejudice, bigotry, and tyranny.