On September 7, 1820, Lord Cochrane's fleet dropped anchor in a bay near Pisco, one hundred and fifty miles south of Lima. San Martin's army, numbering four thousand five hundred Argentines and Chileans, disembarked without opposition and occupied the fertile, vine-covered valleys. To undertake a campaign for the conquest of Peru with such a force seemed absurd. The viceroy's troops were five times as numerous; at Lima alone he had nearly nine thousand men; as many more were quartered at Cuzco, Jauja, and Arequipa, besides the six thousand veterans who guarded the Bolivian frontier against an invasion from Buenos Aires. The contest was, however, really not so unequal as it appeared. The Spanish armies were made up of native Peruvians and Bolivians, with some Venezuelans; sympathisers with the patriot cause swarmed in their ranks; many were waiting an opportunity to desert; the viceroy had little control over his generals, and the arrival of the Argentine army stimulated the activity of the patriot societies in the Peruvian cities.
From Pisco San Martin detached a force of twelve hundred men under the command of General Arenales, which ascended the Cordillera, roused the population of the plateaux immediately back of Lima, and defeated a detachment under General O'Reilly near Cerro de Pasco. The Indians rose, but when the Spaniards came up in force Arenales retired to the coast, leaving his allies to be mercilessly slaughtered. Meanwhile San Martin with the main body had taken ship at Pisco and, sailing north, landed at Huacho, seventy miles beyond the capital. His three thousand men could not hope to succeed in a direct attack on the city, defended by thrice that number of disciplined troops. On the other hand, the Spanish army was shut off from the sea; its base was now far back in the interior; its line of communication might be cut at any moment by other expeditions like that of Arenales. Lima and the coast towns were decidedly disaffected. San Martin's plan was to wait patiently until a rising should compel the Spaniards to retire to the interior, and then to organise the country and gather an army for the final campaign on the plateau. He kept, therefore, at a safe distance from the Spaniards; sent out detachments which scoured the country up to the walls of Lima; and entered into communication with the conspirators in the city. Crowds of young enthusiasts hastened out to join him; Cochrane daringly cut out the frigate _Esmeralda_ under the very guns of Callao castle; an expedition sent to Tacna, on the extreme southern coast, was enthusiastically received; and numerous desertions from the Spanish army culminated in a battalion of Venezuelans coming over in a body. The viceroy was sorely puzzled. He hesitated to send his army to attack San Martin, fearing an insurrection or surprise during his absence, and knowing that defeat meant irretrievable ruin. Really only two courses of action lay open to the Spaniards--they must either fight San Martin, and the sooner the better, for he was becoming stronger every day--or they must abandon Lima and concentrate on their base in the mountains. The viceroy could not make up his mind to abandon the ancient capital, and he was reluctant to expose his family to the hardships of a guerilla warfare in the mountains.