While the assault of the fortress was pushed with the utmost fury and was seen from the island of Quiriquina, the remainder of the Spanish army came over to the aid of the garrison, and formed in order of battle.  The debarkation was observed by Caupolican who immediately sent a part of his troops to meet this new enemy.  After a severe conflict of several hours, this detachment was driven back to the mountain with heavy loss, so that the Araucanians were now placed between two fires; yet they did not lose courage, and continued fighting till mid-day.  At length, worn out with the length of the combat, the Araucanian general drew off to the Biobio, determined to collect a new army and to return to the attack.  Having in a short time reinforced his army, Caupolican began his march towards Conception; but, learning on his way that the governor had received a numerous reinforcement, he halted on the banks of the Biobio, deeply chagrined at not being able to effect the destruction of the new fortress of Conception, which had been twice performed by Lautaro with the universal applause of the nation.

In fact, on the preceding day the Spanish cavalry from Peru, consisting of 1000 well armed men, had arrived at Conception, together with another squadron of Spanish horse from Imperial, and 2000 Promaucian auxiliaries.  Being now at the head of a numerous and well-appointed army, Don Garcia determined to invade the Araucanian territory.  For this purpose he crossed the Biobio in boats, six miles above its mouth, where the river is about 1500 paces broad.  As the Spanish cannon in the boats commanded the opposite bank of the river, Caupolican made no attempt to obstruct the passage, but drew up his army at no great distance in a position flanked by thick woods, by which his retreat would be secured in case of being defeated.  The battle began by several skirmishes, which ended in favour of the Araucanians; several advanced parties of the Spaniards being repulsed by the enemy with loss, though reinforced by order of Ramon the quarter-master-general.  Alonzo Reynoso likewise, who was dispatched to their aid with fifty horse, was defeated in his turn, and obliged to retreat leaving several of his men dead on the field.  At length the two armies met and joined battle.  Encouraged by the advantages they had already gained, the Araucanians used every effort to come to close quarters with the Spaniards, notwithstanding the heavy fire of eight pieces of artillery which played incessantly from the front of the enemy.  But when they came within reach of the musquetry, they were quite unable to resist the close and well directed fire continually kept up by the veteran troops of Peru.  After many ineffectual attempts to close in with the Spaniards, and losing a vast number of their bravest warriors, they fell into confusion from the vacancies in their ranks, and began to give ground.  By a well timed charge, the cavalry put them completely to the rout, and made a prodigious slaughter among them in their flight to the woods.