The Union Army was the land force that fought for the Union during the American Civil War. It was also known as the Federal Army, the U.S. Army, the Northern Army and the National Army. It consisted of the small United States Army (the regular army), augmented by massive numbers of units supplied by the Northern states, composed of volunteers as well as conscripts.
The Union Army was composed of numerous organizations, which were generally organized geographically.
An organization that covered a defined region, including responsibilities for the Federal installations therein and for the field armies within their borders. Those named for states usually referred to Southern states that had been occupied. It was more common to name departments for rivers or regions.
District: A subdivision of a Department . There were also Subdistricts for smaller regions.
A collection of Departments reporting to one commander . Military Divisions were similar to the regions described by the more modern term, Theater.
The fighting force that was usually, but not always, assigned to a District or Department but could operate over wider areas.
(such as Department of the Tennessee, Department of the Cumberland) (Department of the Pacific, Department of New England, Department of the East, Department of the West, Middle Department)
District== (e.g., District of Cairo, District of East Tennessee)
(e.g., Military Division of the Mississippi, Military Division of the Gulf)