One of the main priorities of the Chilean command in Lurín was to reconnoiter the opposing camp, since there was little information on the positions that the Peruvian forces occupied in San Juan and in other sectors for the defense of Lima.

The Chilean command needed to know as much as possible the characteristics of these positions in order to adopt a battle plan, so the exploration missions were entrusted to several officers who were given a few troops to carry out these operations, although some were executed by Baquedano himself in the company of his General Staff.

The Chilean Minister of War, José Francisco Vergara defended the idea of carrying out an attack through Ate, along the Manchay road to surround the Peruvian army by a flank movement and interpose between it and the capital.

General Baquedano, considering the possibility of the encircling attack proposed by War Minister Vergara, commissioned Colonel Barbosa to explore the Manchay and Ate road, giving him a strong division of approximately 2,000 men from the three arms.

His objective was to make a reconnaissance along the road called Cieneguilla, enter the valleys of Lima or Ate and approach the defensive system of the San Bartolomé Hill, located southwest of the Peruvian capital.

At about 5 AM, Barbosa's division entered Pampa Grande and then ran into a "cajon" or portezuelo called Rinconada de Ate, where a Peruvian defensive position was blocking the road to the valley.

The chief of the Chilean General Staff refers to the arrival of Barbosa's division at the Peruvian position in Rinconada de Ate and to the combat in general as follows: Before going down to the valley, that force found the road blocked by a considerable number of automatic mines that covered the field and exploded under the feet of the troops, at the same time that some enemy guerrillas fired, sheltered behind a triple trench of ditches, which cut the width of the open through which the road turns, while others crowned the heights on either side.The enemy cavalry appeared in the valley behind the infantry, and the cannons of the forts to the south of Lima fired grenades at our ranks.The Peruvian position was strong enough to resist a frontal attack, but it could be taken if the slopes on the sides were dominated or an attack was made from the rear making an encircling movement through the hills.

He further ordered that the Horse Grenadiers begin an advance to the left of the Peruvian position so that they would then make a detour through the hills and take the rearguard while the rest of the Chilean troops remained in the reserves.

Colonel Vargas, who understood the fatality of this maneuver for his forces and the futility of engaging in resistance, decided to retreat to the fortified position of Vásquez before the Chilean troops entered the trench.

At that time, the cavalry section led by Lieutenant Colonel Millán appeared to protect the retreat of Vargas's forces, which at the beginning of the Chilean attack was more than half a league from Rinconada.

Valuable information obtained by the Chileans was achieved with the capture of an administrator of a hacienda in the Ate Valley, an American engineer named Murphy, who gave important scientific details about the Peruvian lines that defended the capital.