Marco, the Spanish governor, had not had sufficient time to concentrate his scattered regiments since the first news had come that San Martin was coming in force by the northern passes. Of his five thousand men only two thousand were able to get between San Martin's advance and Santiago. The Argentine general was sure of having the largest numbers at the point of conflict, but the Spanish troops were veterans of the Peninsula and were commanded by a skilful and resolute general. He concentrated his force in a strong position in a valley on the south side of the transverse range that separates Santiago from the Aconcagua valley. He had hoped to make his stand at the top of the pass, there four thousand feet high, but San Martin had been too quick for him. However, the position was admirable for a stubborn defence. The highroad to Santiago descended from the pass down a narrow valley, which, just in front of the Spanish position, opened into a larger valley running at right angles. The artillery of the Spaniards commanded the narrow mouth of the upper valley, and on a side hill there was room to deploy the infantry and cavalry. The Argentine troops would be enfiladed in the close gut before they could form in line of battle. San Martin employed the tactics of the Persians at Thermopylae. There was an abandoned road running over the summit a little to the west of the travelled route and debouching into the same valley a little below the Spanish position. Through this O'Higgins, the chief of San Martin's Chilean allies, at two o'clock in the morning of February 12th, started with eighteen hundred men. By eleven he had reached the main valley and turned up it to attack the Spaniards on their left flank. His first assault, made without waiting for the other division to come down in front, was repulsed. San Martin, sitting on his war-horse on the heights above, galloped down the slope, leaving orders to hasten the descent of the main body. As he reached the lower ground and joined the Chileans, he saw the head of his main column appear through the mouth of the pass. O'Higgins again attacked, and the Spaniards, taken in flank and with their centre assailed in _echelon_ by the Argentine squadrons and battalions, were at a hopeless disadvantage. The position of their infantry was carried by the bayonet, while the patriot cavalry charged the artillery and sabred the men at their guns. The infantry were the flower of the Spanish regulars; they formed a square and for a time held their stand. Finally, surrounded on three sides, their artillery gone, and fighting against double their number, they broke and retreated over the broken ground in their rear. Less than half escaped and a quarter were killed on the field and in the pursuit. The patriots lost only twelve killed and one hundred and twenty wounded.