Both San Martin and O'Higgins had perceived that the only effective way to eliminate the Peruvian wedge was to gain control of its approaches by sea.

The Chileans had already won some success in this direction when the fiery and imperious Scotch sailor, Thomas Cochrane, Earl of Dundonald, appeared on the scene and offered to organize a navy.

At length a squadron was put under his command.

With upwards of four thousand troops in charge of San Martin the expedition set sail for Peru late in August, 1820.
While Cochrane busied himself in destroying the Spanish blockade, his comrade in arms marched up to the very gates of Lima, the capital, and everywhere aroused enthusiasm for emancipation.

When negotiations, which had been begun by the viceroy and continued by a special commissioner from Spain, failed to swerve the patriot leader from his demand for a recognition of independence, the royalists decided to evacuate the town and to withdraw into the mountainous region of the interior.