Led by General P. G. T. Beauregard—the hero of Fort Sumter—the 30,000-odd Confederates were poised in a defensive position near the stream called Bull Run, which meandered through the fields to flow into the Potomac. Three miles behind them was Manassas Junction, the most important railroad connection in this part of the Southern states. Holding that railway link was vital to the Southern cause.

Another 20,000 or so Confederates were in the Shenandoah River Valley, about eighty miles away, but they were shadowing, and being shadowed by, a Union force of almost 30,000. Although we know a lot of the numerical strengths and weaknesses, they were not always apparent at the time, not even to the generals on the ground. This led to a lot of false numbers being circulated and a lot of false claims after the first battle was fought.