With all the hoople surrounding illegal immigration, you may not know that the US Government actually *initiated* the importation of Mexican agricultural laborers into California as part of the Bracero Program [wiki] in 1942 to 1964. Congress enacted the Emergency Labor Program, due to the extreme shortage of farm labor workers due to World War II.
Link - from University of California's Calisphere website (with over 150,000 digitized photos, documents, newspaper articles etc. relating to California history).