Rancho Monserate was a 13,323-acre (53.92 km2) Mexican land grant in present day San Diego County, California given in 1846 by Governor Pío Pico to Ysidro María Alvarado. The grant extended south and east of the present day Fallbrook down to the San Luis Rey River. The grant was bounded on the west by Pico's Rancho Santa Margarita y Las Flores.
Ysidro María Alvarado (1811–1863), son of Francisco Xavier Alvarado (1766–1831) and Maria Ygnacia Amador (1770–1851), married Maria Micaela Avila (1816–1845) in Los Angeles, California. Shortly after bearing three children, Micaela died from unknown causes, and Alvarado married her sister Manuela Lorenzo Avila.
With the cession of California to the United States following the Mexican-American War, the 1848 Treaty of Guadalupe Hidalgo provided that the land grants would be honored. As required by the Land Act of 1851, a claim for Rancho Monserate was filed with the Public Land Commission in 1853, and the grant was patented to Ysidro María Alvarado on July 17, 1872.
A smallpox epidemic began in Mission San Juan Capistrano in 1862 and appeared to be spreading south toward San Diego. To keep them safe, Alvarado sent his children to Los Angeles to stay with their uncle, Francisco Avila. The disease raged through the rancho, killing Ysidro Alvarado and his wife in 1863 along with 21 ranch hands and domestic servants. After a number of smallpox victims were interred at Mission San Luis Rey, Colonel Cave Johnson Couts of Rancho Guajome, at that time owner of the mission grounds, made it known that he would not allow any further burials. However, on January 13, 1863, the family and friends of Ysidro Alvarado chose to honor his dying wish and bury him at the mission graveyard. Tomas Alvarado, present at the ceremony, wrote that, as dirt was being thrown on the casket of Ysidro Alvarado, Couts's brother William Blount Couts and two of his trusted men challenged the mourners from the wall of the graveyard, holding guns and shouting "Como diputado del Sherif del condado, no es permitido que este Señor se entierra aqui" ("As deputy of the sheriff of the county, [I say] it is not permitted to bury this man here.") Couts was aiming a double-barreled shotgun at them, and Leon Vasquez of the burial party picked up a shovel and ran toward the threat. Couts fired one barrel and missed Vasquez. As Vasquez jumped onto the wall, Couts shot him in the face and killed him. The rest of the unarmed burial party scattered in fear of their lives as more shots were fired at them, wounding two. Colonel Cave Couts explained the incident the next day, saying "In avoiding the loathsome disease now infesting our community, we have had to resort to arms, resulting in the killing of one man." He defended his brother's actions, writing to his lawyer that Vasquez "is really not worth noticing ... He is known as a bad character." William Blount Couts was charged with murder but, despite depositions from eight eyewitnesses, succeeded in having the charges dropped because of paperwork technicalities.