The Department of the Pacific was first organized at San Francisco, California October 31st 1853. It reported to Army headquarters in Washington, DC and oversaw the military in California and the Oregon and Washington Territories. It moved its headquarters to Benicia, California for a short time in 1854, but was back in San Francisco by 1857. In 1858 the Utah Territory was included this department. The Department was disbanded on September 13th 1858.
On January 15th 1861, with the Civil War about to start, the Union Army reorganized the Department of the Pacific. The first commander was Union Colonel Albert Sidney Johnston. He served as commander until March 1861, when Johnston resigned his commission to join the Confederate Army. He was replaced by Union Brigadier General Edwin Vose Sumner.