San Diego County is a county located in the southwestern corner of the U.S. state of California. It is the most southwestern county in the 48 contiguous United States. Its county seat and largest city is San Diego. According to the 2010 U.S. Census, San Diego County had a population of 3,095,313 people, making it the second most populous county in California- first being Los Angeles County.
San Diego County has of coastline. It has a mild Mediterranean to semi-arid climate. There is also 16 significant naval and military locations of the United States Navy, U.S. Marine Corps and the U.S. Coast Guard, including Naval Base San Diego, Marine Corps Base Camp Pendleton, Marine Corps Air Station Miramar, and Naval Air Station North Island.
San Diego County defines the metropolitan statistical area of San Diego-Carlsbad-San Marcos, and in its metropolitan capacity as Greater San Diego. San Diego County is also part of the San Diego–Tijuana metropolitan area. This area, having about five million people, is the largest bi-national metropolitan area shared between the United States and Mexico.
Arising from an effort by the state government to identify regional economies, San Diego County and Imperial County are part of the Southern Border Region, one of nine such regions. As a regional economy, the Southern Border Region is the smallest but most economically diverse region in the state. However, the two counties maintain weak relations and have little in common aside from their border proximity.
From north to south, San Diego County extends from the southern borders of San Clemente (Orange County) and Temecula (Riverside County) to the US-Mexico International Border and Tijuana. From east to west, San Diego County stretches from the Pacific Ocean to Imperial County.
European settlement in what is now San Diego County began with the founding of the San Diego Presidio and Mission San Diego de Alcalá by Spanish soldiers and clerics in 1769. This county was part of Alta California under the Viceroyalty of New Spain until the Mexican revolution. From 1821 through 1848 this area was part of Mexico.
San Diego County became part of the United States as a result of the Treaty of Guadalupe Hidalgo in 1848, ending the U.S.-Mexican War. This treaty designated the new border as terminating at a point on the Pacific Ocean coast which would result in the border passing one Spanish league south of the southernmost portion of San Diego Bay, thus ensuring that the United States received all of this natural harbor.
San Diego County was one of the original counties of California, and it was created at the time of California statehood in 1850. San Diego County was named for San Diego Bay, which had been renamed in 1602 by Sebastián Vizcaíno in honor of the Franciscan St. Didacus of Alcalá, known in Spanish as San Diego de Alcalá de Henares, and whose name was borne by Vizcaíno's flagship.
At the time of its establishment in 1850, San Diego County was relatively large, and included all of southernmost California which was south and east of Los Angeles County. As such it included areas of what are now Inyo County and San Bernardino County, as well as all of what is now Riverside County and Imperial County.
During the later part of the 19th century, there were numerous changes in the boundaries of San Diego County, when various areas became separated for the counties mentioned above. The most recent changes were the establishments of Riverside County in 1893 and Imperial County in 1907. Imperial County was also the last county to be established in California, and after this division, San Diego no longer extended from the Pacific Ocean to the Colorado River, and it no longer covered the entire border between California and Mexico.