To the Senate of the United States:
In compliance with the resolution of the Senate of the 28th ultimo, requesting information as to the number and designations of military departments formed since the 1st day of August, 1867, and as to the statute or other authority under which they have been established. I transmit a report from the Adjutant-General's Office showing the organization since that date of the Department of Alaska and the Military Division of the Atlantic.
The orders issued by me upon this subject are in accordance with long-established usage and hitherto unquestioned authority. This will be readily seen from the accompanying report, which shows that, employing the authority vested by the Constitution in the President as Commander in Chief of the Army, it has been customary for my predecessors to create such military divisions and departments as from time to time they deemed advisable.
Andrew Johnson, Special Message Online by Gerhard Peters and John T. Woolley, The American Presidency Project https://www.presidency.ucsb.edu/node/202507