Image of Irvin McDowell via Wikipedia
On July 16, 1861, the Bull Run campaign began when Union General Irvin McDowell moved his Army of Northeastern Virginia out of Washington area, southeast into Virginia toward General P. G. T. Beauregard's Confederate Army of the Potomac. The advance would lead to the first major battle of the Civil War, the First Battle of Bull Run, on July 21.
McDowell's army was the largest ever assembled in American history to that point, some 35,000 men. Facing him, Beauregard had 20,000 encamped at Manassas Junction behind a small stream called Bull Run.
Two other armies complicated the campaign. Union General Robert Patterson's Army of the Shenandoah had crossed the Potomac and moved south to confront General Joseph E. Johnston's army, also known as the Army of the Shenandoah. Patterson's vague orders called for him to keep Johnston busy in the Shenandoah Valley and prevent him from moving east and reinforcing Beauregard. If Joe Johnston could reach Beauregard with his 12,000 men, the odds would be a lot more even.
McDowell was being pushed into a battle that he didn't think his army was ready for. He wanted more time to organize and train them, but he was under intense time constraints. Almost everyone, North and South, thought this would be a short war. The Northern public especially was clamoring for an advance on Richmond and a decisive battle that would win the war.
Another time constraint: McDowell's army (and Patterson's too) would soon be unraveling. A product of the belief in a short war, the first wave of Union volunteers after the firing on Fort Sumter were signed to three-month enlistments. Now the three months was almost up. If McDowell was going to use this army, he would have to do it soon.
McDowell wanted more time, expressing the need for more training for this still green army. Lincoln had tried to reassure him, saying, "You are green, it is true, but they are green also; you are all green alike." This was true enough, but the Confederates weren't having to make a grueling march to get to the battle. Lincoln had directed McDowell to move out by July 9. Arrangements had taken longer than expected, but McDowell was finally on the road a week late.
And as McDowell was beginning to move, Johnston was parrying with Patterson in the valley, sending J. E. B. Stuart's cavalry against Patterson's advance units and forcing the old general to sit this one out. Johnson would soon slip away, but would not reach Beauregard until July 20-21.
McDowell should have had enough time to reach Beauregard and give battle before Johnston showed up, but his misgivings about his army would be realized as soon as they were on the move.
The advance was a mess. The men had never marched in formation for hours in July heat with full gear, and officers had never led men in a long march. It was a vastly different experience than marching for an hour or so around a parade ground. Bottlenecks and long halts quickly developed. By the evening of the 16th, McDowell's advance was in Fairfax Court House, where they drove off enemy skirmishers and made camp. The next day, the troops were exhausted and could only cover six miles. It took until July 18 for the advance to reach Centreville, close to the Confederate position, and would take another two days for McDowell to get the rest of his army up. He would finally be ready for an attack on July 21.