The Potomac River helped dictate the pace of campaigning for the Union and Confederate armies. Running west to east, and with its headwaters extending beyondin the lower , the river proved to be a major obstacles for attempting to maneuver their armies. During Confederate general ‘s invasions of the North—the in 1862 and the in 1863, the need to secure fords across the Potomac River helped dictate a western strategic route, across the river at Boteler’s Ford, , Harpers Ferry, and Williamsport. The presence of the Army of the Potomac east of the Blue Ridge Mountains meant that eastern crossings of the Potomac were protected. On the whole, however, because the main theater of war in Virginia had shifted south after Confederate general withdrawal from Manassas Junction early in 1862, the Potomac played little role in land campaigning.
As Johnston pulled back toward Orange County and Fredericksburg, Union generalsought a way to flank him. Beginning in March 1862, he loaded his Army of the Potomac on transports and sailed it down the Potomac River and into the Chesapeake Bay, landing at on the tip of the Middle Peninsula. From there, McClellan marched up the Peninsula only to be stopped by Johnston during the and then Lee in the before ever reaching the Confederate capital at . Although it failed, McClellan’s campaign would not have been possible without the Potomac River, whose major role in the war had now shifted to the maritime sphere.
In 1861, Confederates—perhaps anticipating something like the Peninsula Campaign—had attempted to deny Union forces use of the Potomac, erecting several batteries along the river’s south shore and firing on Union ships. Largely ineffective, these batteries nevertheless proved an embarrassment to the North because they restricted access to the capital only to river traffic willing to risk the possibility of a lucky cannon shot. Once Johnston retreated and the conflict moved south, the Confederate guns withdrew and normal traffic resumed. Still,continued to harass boats along the river for the rest of the war.
Despite worries in 1862 that the Confederate ironclad warshipmight sail up the Chesapeake, and then the Potomac, to threaten Washington, D.C., the Union navy achieved superiority over its Confederate foes in the Chesapeake. With the destruction of CSS Virginia in 1862, the Confederate naval threat had been ended, with the exception of the , which lurked safely above Confederate river fortifications at Drewry’s Bluff. This enabled military planners to use the Potomac River to supply Union armies operating in Virginia. For instance, while he commanded the Army of the Potomac late in 1862, located his main supply depot at Aquia Creek, where waterborne supplies could be offloaded onto trains and carried along the Richmond, to Union forces campaigning along the Rappahannock River. In the spring of 1864, during the Overland Campaign, Union general-in-chief placed his supply depot at Belle Plain near Fredericksburg, before shifting it to a base along the Rappahannock.