After the Battle of San Pascual, the loyalist Californio lancers whom belonged to the Mexican military force that took part in the battle, broke up into different groups. A group of eleven men traveled to Rancho Pauma, owned by Jose Antonio Serrano. Along the way, the men stole a herd of fine horses belonging to the Pauma band of the Luiseno Indians and retreated to the rancho.
Serrano, his fourteen-year-old son Jesus, and his brother-in-law Jose Aguilar had gone to a nearby settlement called Pala, where Serrano's wife and other children were staying. Before leaving, Serrano was said to overhear two Luiseno women discussing a plot to capture the Californios.
In the evening, Chief Manuelito Cota (also called Chief Manuel) took a group of the Pauma band with him to Rancho Pauma. The chief knocked on the door and introduced himself. The Californios knew the chief and had peaceful relations with him. When the Californios opened the door, the chief and his men captured the lancers.
They took eleven prisoners to El Potrero, an Indian rancheria, for the night. The next day the party traveled to Aqua Caliente (Warm Springs, now known as Warner Springs). Chief Manuel called the area bands together for a tribal council to decide the fate of the horse thieves. Most tribal chiefs believed the prisoners should be scarred and then released. William Marshall, a local American, entered the debate. He swayed the vote in favor of executing the prisoners.(See Notes below.)
The tribal leaders decided the Californios should undergo ritual torture and execution for their crimes against the Pauma band. After the Californios witnessed the first execution, the men wept and begged for their lives. Santos Alipas, thirteen years old, stood up and reportedly said, What's the use of crying? We can only die once; let us die like brave men. Impressed with Alipas' bravery, the Luisenos gave him a choice on how he wanted to die. The boy asked to be shot. He was immediately shot through the forehead.
When word of the massacre reached Mexican forces in Los Angeles, General Jose Maria Flores designated Jose del Carmen Lugo of San Bernardino to lead a group of soldiers to avenge the lancers' deaths. The soldiers carried out their mission with the aid of allied Cahuilla Indians. They killed an estimated 33-125 Luiseno warriors, including the leaders who had authorized the earlier attack on the Californios. The action was called the Temecula Massacre.
Those killed in Pauma were: Manuel Serrano, Ramon Aguilar, Francisco Basualdo, Jose Maria Alvarado, Mariano Dominguez, Santiago Osuna, Jose Lopez, Estaquio Ruiz, Juan de la Cruz, Unnamed New Mexican and Santos Alipas.*Notes: According to Herbert Lockwood, William Marshall at one time was in love with Senorita Osuna. She told Marshall to leave her alone, as she planned to marry José María Alvarado. Marshall was heartbroken and angry. He moved to Warner's ranch where he was hired as a foreman. He married a Cupeno woman and was accepted as an unofficial member of her tribe. When he saw Alvarado among the Californio prisoners, he saw an opportunity to take his revenge. Some historians believe that Marshall had no part in the Pauma Massacre.