It oversaw the military affairs in the country west of the Mississippi River to the borders of California and Oregon.
The Department of the West was created in a reform of army organization nationwide on October 31, 1853, from a consolidation of the existing 6th Military District (headquartered at Jefferson Barracks, Missouri) and 7th Military District (Fort Smith, Arkansas) Departments.
The Department of the West's headquarters continued at Jefferson Barracks in St. Louis, although it moved briefly to Fort Leavenworth, Kansas, during the Bleeding Kansas skirmishes.
On June 6, 1861, Missouri was transferred to the Department of the Ohio.
Frémont was relieved by Maj. Gen. David Hunter under the orders of President Abraham Lincoln, contingent upon the circumstance that Frémont did not lead an army into battle by October 24, 1861.