Oceanside is a coastal city located on California's South Coast. It is the third-largest city in San Diego County, California. The city had a population of 183,095 at the 2010 census. Together with Carlsbad and Vista, it forms a tri-city area. Oceanside is located just south of Marine Corps Base Camp Pendleton. Originally inhabited by Native Americans, the first European explorers arrived in 1769. Spanish missionaries under Father Junipero Serra founded Mission San Luis Rey de Francia on a former site of a Luiseo Indian village on the banks of the San Luis Rey River. In the early 19th century, the introduction of farming and grazing changed the landscape of what would become Oceanside. The area - like all of California - was under Spanish, then in 1821 under Mexican rule, and conquered by the U.S. in 1848.