United States Armies in the East
The Department of Washington was established on April 9, 1861. It consisted of the District of Columbia including its original territory of Alexandria County, Virginia, and the State of Maryland as far as Bladensburg.The Department was charged with the defence of the national capitol and served as the assembly point for the thousands of volunteers pouring in from all over the North to build an army to end the rebellion.
 Lt. Colonel Charles Smith |
 Colonel Joseph Mansfield |
While the creation of the Department of Northeastern Virginia temporarily made the Department of Washington a rear area sending supplies and reinforcements to McDowell, the defeat at Bull Run returned Washington to the front lines. Within days the Department was consolidated into what was to become the Army of the Potomac.
Organization of the Department of Washington
1861: April • May • June • July
Reestablished
1863: February, March, April, May, June, July, August, September, October, November, December
1864: January, February, March, April, May, June, July, August, September, October, November, December
1865: January, February, March, April, May, June, July, August
Timeline of the Department of Washington
| April 9 | Department established under Lieutenant Colonel Charles F. Smith |
| April 27-28 | Colonel Joseph K. F. Mansfield took over the department from Smith |
| May 24 | Union troops under New York Militia General Charles W. Sandford cross the Potomac from Washington and occupied Alexandria, Virginia, to reduce the threat to Washington D.C. |
| May 27-28 | The Department of Northeastern Virginia was created and assigned to Brigadier General Irvin McDowell. |
| July 21 | The Federal defeat at the Battle of Bull Run returned Washington to the front line of the war. |
| July 25 |
Department was consolidated with the Department of Northeastern Virginia and the Department of Maryland to form the Military District of the Potomac. |
| August 17 | Merged into the Department of the Potomac |
| February 2, 1863 | The Department of Washington was reestablished under the command of Major General Samuel Heintzelman, split away from the Department of the Potomac when Joseph Hooker took over that department from Ambrose Burnside. It consisted of the area north of the Potomac River in Maryland from Piscataway Creek to Annapolis Junction, to the mouth of the Monocacy River, and south by Goose Creek and the Bull Run mountains to the mouth of the Occoquan River. |
| October 1863 | Major General Christopher Augur took command |
| June 21, 1864 | The department was redefined as that part of Maryland south of a line drawn from the mouth of the Monocacy River to Annapolis Junction, and west of the Patuxent River, the District of Columbia, and the area by Goose Creek and the Bull Run mountains to the mouth of the Occoquan River. |
| August 7, 1864 | The department was attached to the Middle Military Division. |