Executive Order—President's Special War Order No. 3
Major-General McClellan having personally taken the field at the head of the Army of the Potomac, until otherwise ordered he is relieved from the command of the other military departments, he retaining command of the Department of the Potomac.
Ordered further, That the departments now under the respective commands of Generals Halleck and Hunter, together with so much of that under General Buell as lies west of a north and south line indefinitely drawn through Knoxville, Tenn., be consolidated and designated the Department of the Mississippi, and that until otherwise ordered Major-General Halleck have command of said department.
Ordered also, That the country west of the Department of the Potomac and east of the Department of the Mississippi be a military department, to be called the Mountain Department, and that the same be commanded by Major-General Fremont.
That all the commanders of departments, after the receipt of this order by them, respectively report severally and directly to the Secretary of War, and that prompt, full, and frequent reports will be expected of all and each of them.
Abraham Lincoln, Executive Order—President's Special War Order No. 3 Online by Gerhard Peters and John T. Woolley, The American Presidency Project https://www.presidency.ucsb.edu/node/202530