The natives observed the progress of this new settlement with much jealousy, and concerted measures for freeing themselves from such unwelcome intruders; but, as Valdivia discovered their intentions, he confined the chiefs of the conspiracy in his new fortress; and having intimation of a secret intelligence being carried on between the Mapochians and their neighbours, the Promancians, he repaired with a body of sixty horse to the river Cachapoal or Rapel to watch the motions of that brave and enterprising nation.  This precaution was however altogether unnecessary, as that fearless people had not sufficient policy or foresight to think of uniting with their neighbours in order to secure themselves from the impending danger.  Taking advantage of the absence of Valdivia, the Mapochians fell upon the new settlement with desperate fury, burnt all the half-built houses, and assailed the citadel on all sides, in which the inhabitants had taken refuge.  While the Spaniards were valiantly defending their imperfect fortifications, a woman named Inez Suarez, beat out the brains of all the captive chiefs with an axe, under the apprehension that they were endeavouring to regain their liberty, and might assist the assailants in gaining possession of the fort.  The attack began at day-break, and was continued without intermission till night, fresh assailants continually occupying the places of those who were, slain or disabled.

The commandant of the Spaniards, Alonzo de Monroy, found means to send a messenger to inform Valdivia of his situation; and the governor accordingly hastened to the aid of the besieged with all possible expedition, and found the ditch almost filled with dead bodies, while the enemy, notwithstanding the heavy loss they had sustained, were preparing to renew the assault.  Drawing out its infantry from the fort to join the cavalry he had along with him, Valdivia advanced in order of battle against the forces of the enemy, who were posted on the bank of the Mapocho.  The battle was again renewed in this place, and obstinately contested with equal valour on both sides; but with great disadvantage on the part of the natives, who were far inferior in arms and discipline to the Spaniards.  The musquetry and the horse made a dreadful slaughter among Mapochians, who were only armed with bows and slings; yet obstinately bent upon preserving their independence, and regardless of their own importance, they rushed on to inevitable destruction; till having lost the flower of their valiant warriors, and reduced to a small number, they at length fled and dispersed over the plain.  Notwithstanding this memorable defeat, and others of not less importance which they sustained afterwards, the Mapochians did not cease for the space of six years to keep the Spaniards closely blockaded in St Jago, continually attacking them on every opportunity, and cutting off their provisions so effectually, that they were often reduced to great straits, having to subsist upon unwholesome and loathsome viands, and what little grain they were able to raise under protection of the cannon from the ramparts.  At length, worn out and brought to utter ruin by this incessant warfare, the remnant of the Mapochians destroyed their own crops and retired to the mountains, leaving the fertile plains around the new city utterly deserted and uncultivated.