The mid-century discovery of gold in California led to the sudden growth and importance of San Francisco, and prompted the U.S. government to establish a military reservation here. By executive order, President Millard Fillmore established the Presidio for military use in November 1850. During the 1850s and 1860s, Presidio-based soldiers fought Native Americans in California, Oregon, Washington, and Nevada. The outbreak of the American Civil War in 1861 re-emphasized the importance of California's riches and the military significance of San Francisco's harbor to the Union. This led, in 1862, to the first major construction and expansion program at the Presidio since its acquisition by the United States.