Its goal was to protect the state capital and the southern portions of the commonwealth of Pennsylvania, and to deny the Confederate army passage across the vital Susquehanna River.
New York troops serving under the Department of the Susquehanna were first engaged in a skirmish with Confederate cavalry under Albert G. Jenkins at Greencastle on June 20, losing one man killed (considered the first casualty of the Gettysburg Campaign on Northern soil).
Couch designated his aide-de-camp, Major Granville O. Haller, as the sector commander to defend Adams and York counties, with regional headquarters in Gettysburg.
On June 26, 1863, advancing Confederates under Jubal Early and John Brown Gordon routed Haller's militia at Gettysburg and occupied the borough.
The troops of the department assigned to General Smith took part in skirmishes against elements of Richard S. Ewell's corps in Cumberland County at Sporting Hill on June 30 and against J.E.B.
Couch dispatched Smith's men, along with many of Haller's, to help George G. Meade pursue Robert E. Lee's retreating Army of Northern Virginia.
In January 1864, Couch responded with troops to rumors that a band of disgruntled constituents planned to prevent the inauguration of Pennsylvania Governor Andrew Curtin.