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.

The soldiers under Valdivia became wearied and disgusted by this continual war, so different from what they had expected; and as they believed him obstinately bent upon adherence to his own plan, and resolved to continue the settlement in spite of every opposition from the natives, they entered into a conspiracy to kill their general and to return into Peru, where they expected to enjoy more ease and tranquillity. Having fortunately got notice of this conspiracy, Valdivia, who possessed great prudence and an insinuating address, soon conciliated those who were least implicated. After this, as he only had the title of general which did not confer any civil and judicial power, he assembled the Cabildo of the city, and persuaded them to invest him in the office of governor of the city and kingdom. In this imposing capacity, he tried and capitally punished some of the ringleaders of the conspiracy, and then prudently exerted himself to soothe the turbulent and seditious spirits of the remainder, by buoying up their hopes with the most flattering promises of future wealth. He had often heard in Peru, that the valley of Quillota abounded in mines of gold, and was hopeful therefore of being able to obtain a sufficient quantity from thence to satisfy the avidity of his soldiers. Notwithstanding the difficulties with which he was surrounded, he sent a party of soldiers into the valley of Quillota, with orders to superintend and protect a number of labourers in digging for the precious metal said to be abound in that place. The mine which was opened upon this occasion proved remarkably rich and productive, surpassing their most sanguine hopes; so that all their past sufferings and present difficulties were soon buried in oblivion, and henceforwards no one had the remotest wish to leave the country. Valdivia, encouraged by this success to new enterprises, ordered a carrack or ship of some considerable size to be built at the mouth of the river Chillan, which traverses the valley of Quillota, for the purpose of more readily obtaining succours from Peru, without which he was fully sensible he could not possibly succeed in the vast enterprise he had in view, which was no less than to accomplish the entire reduction of Chili.

In the mean time, considering the urgent state of his affairs, Valdivia resolved to dispatch two of his principal officers, Alonzo Monroy, and Pedro Miranda by land to Peru, with an escort of six horsemen, whose spurs, bits, and stirrups he directed to be made of solid gold, hoping thereby to entice a sufficient number of recruits to come to his assistance, by this obvious proof of the riches of the country. Although these messengers were escorted to the confines of Chili by thirty additional horsemen, they were attacked and defeated in the province of Copaipo by a hundred archers, commanded by Coteo, an officer of the Ulmen of that province. Of the whole party none escaped with life but the two officers, Monroy and Miranda, who were made prisoners and carried before the ulmen covered with wounds. The prince had resolved on putting them both to death; but, while deliberating on the mode of execution, his wife, the ulmena or princess of Copaipo, moved by compassion for their unhappy situation, successfully interceded with her husband to spare their lives, unbound them with her own hands, tenderly dressed their wounds, and treated them as if they had been her brothers. When they were entirely recovered, she desired them to teach her son the art of riding, as several of the Spanish horses had been taken in the late defeat. The two Spaniards readily consented to her request, hoping to avail themselves of this circumstance to give them an opportunity of recovering their liberty, which they did in effect; but the means they employed was marked by a cruel act of ingratitude to their compassionate benefactress, of so much deeper turpitude that it was unnecessary for their purpose. As the young prince was one day riding between them, escorted by a party of archers and preceded by an officer carrying a lance, Monroy suddenly dispatched him with two or three mortal wounds of a poniard. At the same time Miranda wrested the lance from the officer of the guard, who were thrown into confusion by this unexpected event, and the two Spaniards readily accomplished their escape. Being well mounted, they easily eluded pursuit, and made their way through the desert into Peru, whence they continued their way to Cuzco, where Vaca de Castro then resided, who had succeeded to the government after the cruel assassination of Francisco Pizarro by the Almagrian faction.