Department of the Pacific

The Department of the Pacific or Pacific Department was a major command (Department) of the United States Army from 1853 to 1858. It replaced the Pacific Division, and was itself replaced by the Department of California and the Department of Oregon.
Formation.
The Department of the Pacific was created on October 31, 1853, at San Francisco, California, replacing the older Pacific Division, (1848-53) and abolishing the existing 10th (California) and 11th (Oregon) Departments, consolidating them within the new Department. The department reported directly to the headquarters of the Army in Washington, D.C.. It oversaw the military affairs in the country west of the Rocky Mountains (California, Oregon Territory, and Washington Territory), except for the Utah Territory and the Territory of New Mexico east of the 110th meridian west, (thus including most of modern Arizona and southern Nevada). 
On September 2, 1854, the headquarters was moved to Benicia Barracks, in Benicia, California. 
From 1855-57 the Puget Sound District was organized. 
In January 1857, the headquarters again returned to San Francisco. 
On January 14, 1858, the Utah Territory was placed within the Department but soon removed into the Department of Utah, in 1858, that remained until 1861.
Disbandment.
On September 13, 1858, the Department of the Pacific was disbanded, replaced by two new departments: the Department of California and the Department of Oregon. The Department of California included the territory west of the Rockies, the Umpqua and Rogue River districts in Oregon, Utah and New Mexico. The Department of Oregon included the Oregon and Washington Territories.
Reborn in the Civil War.
During the American Civil War the army again reorganized, and on January 15, 1861, the independent Pacific Department was reconstituted by consolidating the Departments of California and Oregon. The first commander of the new Department of the Pacific was Colonel (Brevet Brigadier General) Albert Sidney Johnston who was later to become a prominent General in the Confederate Army.
Districts.
The Department of the Pacific had six subordinate military districts during the Civil War:
Reorganized out of existence.
On July 27, 1865 the Military Division of the Pacific was created under Major General Henry W. Halleck, replacing the Department of the Pacific, consisting of the Department of the Columbia that now consisted of the state of Oregon and the territories of Washington and Idaho and the expanded Department of California that now consisted of the states of California and Nevada and the Territory of New Mexico and Territory of Arizona.
Philippine Expedition.
On 30 May 1898, Gen. Wesley Merritt established in San Francisco the Headquarters, US Expeditionary Forces and Department of the Pacific for the campaign to support Adm. Dewey's forces in the Philippines during the Spanish–American War.
At the end of March 1900, the complexities involved in dealing with the guerrillas and governing the islands led to the transformation of what had been the Department of the Pacific into the Philippine Department with four geographical departments, each of which was, in turn, divided into military districts. This step also brought an end to the Eighth Corps.