He quickly organised the militia of the Archipelago, with the garrison as a nucleus, and crossed to Valdivia with 1,400 men, where he incorporated the garrison of that fortress, raising his force to over 2,000 men. These he arranged in three divisions, each with six guns, and re-embarking, sailed northwards, keeping his destination secret. Three days afterwards, on the 26th March, he landed in the bay of San Vicente, taking the town of Talcahuano in the rear, and threatening Concepcion in front. Talcahuano was taken by assault; the garrison of Concepcion mutinied and gave up the city. Thus speedily he was master of the South, and further strengthened his force by the garrisons of Arauco.

With 2,000 regulars, from 2,000 to 3,000 militia, and with twenty-five guns, he opened the campaign early in April. At Chillán the country rose in his favour, increasing his force to 6,000 men, with whom he occupied the line of the river Nuble, which lies to the south of the Maule.

Carrera was equally active; he proclaimed himself General-in-chief with full powers, declared war against the Viceroy of Peru, set up a gibbet in the Plaza of Santiago, on which to hang all who should hold communication with the enemy, and caused the imposition of a forced loan of two hundred and sixty thousand dollars upon those hostile to the revolution, which measures inspired general enthusiasm and confidence.

On the 1st April he established his headquarters to the north of the Maule, with merely an escort, and gave orders for the concentration of the army at Talca. His friend, Consul Poinsett, accompanied him as a volunteer, and the same day he was joined by O’Higgins, who forgot his resentment, an example followed by Mackenna, who was a talented engineer. Calling in the militia of the South, who remained faithful, in twenty days he was at the head of 10,000 men, from whom he organised an army of 2,500 regulars, badly armed, and as many lancers of the militia, with sixteen field-pieces.

The campaign opened with a piece of good fortune which greatly encouraged the Patriots. It chanced that an officer sent with 500 men to surprise the vanguard of the enemy at a pass on the Maule, misunderstood his orders, and on the night of the 27th April fell in with the main body of the Royalists, some five or six thousand strong. Not knowing who they were, he attacked them and captured the whole of the artillery. At dawn the enemy recovered from their panic, pursued him and recaptured the guns and prisoners. The loss of the Patriots in killed and wounded was four times that of the Royalists, but the moral effect was that of a victory. The greater part of the irregular cavalry deserted from Pareja, who nevertheless advanced to the Maule. The army was drawn up to force the passage, when the men from Chiloe and Valdivia threw down their arms and refused to go further; they cared nothing for the Royalist cause beyond the Maule. Pareja, lying on a stretcher, stricken with a mortal disease, ordered a retreat, on which the rest of the irregulars dispersed, and he was left with little more than a thousand men.

Carrera knew nothing of what had occurred, and let fifteen days pass before he made up his mind to cross the river. The Patriot vanguard under Luis Carrera came up with Pareja on the 15th May, as he was about to pass the Nuble. The Royalists halted, the dying general mounted on horseback for the last time, and placed Captain Sanchez in command. Sanchez at once occupied some rising ground, where he threw up an entrenchment with his baggage, and formed his infantry in square, and opened fire with twenty-seven guns upon the Patriots, checking their advance. Carrera then took the command, and on the arrival of Don Juan José with the second division, drew up his infantry in line with cavalry on the flanks to surround the enemy. Don Juan José, without waiting for orders, attacked the position and was driven back; the same fate befell another battalion which followed his example. The guns were dismounted at the first shot. The cavalry which had passed to the rear of the enemy, were dispersed by artillery fire, and the infantry fell back in disorder. The third division, under O’Higgins and Mackenna, then came up and prevented the advance of the enemy, which would have turned the repulse into a rout. Night put a stop to this strange affair, and Carrera retreated in disorder to San Carlos.

Sanchez crossed the Nuble with all his artillery, without further molestation, and retreated to Chillán, with a loss of six killed and fifteen wounded.

This battle of San Carlos showed that Carrera was destitute of military talent; but he had the strength of mind to reject the councils of his disheartened officers, who advised him to withdraw the army beyond the Maule, and for the first time drew up a definite plan of operations. With one part of his army he occupied Concepcion and Talcahuano, cutting off the retreat of the enemy by sea, and despatched O’Higgins with his division to Arauco, securing the South, but in these manœuvres he lost much time, and one detachment of 650 men left in reserve on the Nuble, was captured by a Royalist force from Chillán.

Sanchez was an obscure soldier, born in Galicia, of no real genius, but quick-sighted, of great tenacity, and devoted to the cause he served. At Chillán he entrenched himself, aided by the people, who were all Royalists, and by the preaching friars, who had there a convent, which soon became a well-provisioned citadel.

When Carrera, against the advice of O’Higgins and Mackenna, determined at the end of July to besiege Chillán, it was already winter, the season of heavy rains. On the 3rd August, Mackenna established a battery of six guns, at four hundred and fifty yards from the trenches. The following morning Sanchez made a vigorous sally but was driven back. The same afternoon he made another attack upon a reserve battery, under the fire of his own redoubts, a ball from which blew up the ammunition of the battery, causing great confusion. Carrera ordered the battery to be abandoned, but his officers disobeyed him, and O’Higgins coming up to the rescue, the enemy was again repulsed.