San Diego County is a county located in the southwestern corner of the State of California. It is the south-westernmost county in the 48 contiguous United States. The City of San Diego is its county seat and by far the largest city in the county. According to the census of 2010, San Diego County had a population of 3,095,313 people, making it the second most populous county in California, following Los Angeles County.
There are also 16 naval and military installations of the U.S. Navy, U.S. Marine Corps, and the U.S. Coast Guard in San Diego County. These include the 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 as 'Greater San Diego'. San Diego County is also part of the San Diego–Tijuana metropolitan area. This area has about five million people and it is the largest 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 common border.
From north to south, San Diego County extends from the southern borders of Orange County and Riverside County to the Mexico–United States border and Baja California. From west to east, San Diego County stretches from the Pacific Ocean to its boundary with Imperial County.
In 1542, the Portuguese-born explorer Juan Rodríguez Cabrillo, sailing for Spain, claimed San Diego Bay for the Spanish Empire, and he named the site San Miguel. In November of 1602, Sebastián Vizcaíno surveyed the harbor and what are now Mission Bay and Point Loma and named the area for Saint Didacus, a Spaniard more commonly known as San Diego. 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 declaration of independence. 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.
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.