The overthrow of the Confederate states proved to be very difficult. The Alleghany Mountains cut the South into two great fields of war. Deep and rapid rivers flowed from the mountains into the Atlantic or into the Mississippi. Each of these rivers was a natural line of defense. The first line was the Potomac and the Ohio. But when the Confederates were driven from this line, they soon found another equally good a little farther south. Then again the South was only partly settled. Good roads were rare, but there were many poor roads. The maps gave only the good roads. By these the Northern soldiers had to march while the Southern armies were often guided through paths unknown to the Northerners, and thus were able to march shorter distances between two battlefields or between two important points.

Northern soldiers crossed the Potomac into Virginia and found the Confederates posted at Bull Run near Manassas Junction. Other Northern soldiers pressed into the Shenandoah Valley from Harper's Ferry. They, too, found a Confederate army in front of them. The plan of the Union campaign is now clear: General McDowell was to attack the Confederates at Bull Run, while General Patterson attacked the Confederates in the Valley, and kept them so busy that they could not go to the help of their comrades at Bull Run.