On July 17, 1861, the advance guard of Union General Irvin McDowell's Army of Northeastern Virginia occupied Fairfax Court House, Virginia, on its way toward its encounter with P. G. T. Beauregard's Army of the Potomac at Manassas. Meanwhile, over in the Shenandoah Valley, Confederate General Joseph E. Johnston began to move away from the Union army in his front, that of General Robert Patterson, to make his way to Manassas to link up with Beauregard.
At Fairfax Court House, advance elements of McDowell's army occupied the town without incident. The Confederates, the advance units of Beauregard's army, evacuated the town just prior to the Federals arrival, leaving behind a quantity of supplies, and settled in just south of the town. Federal cavalry ran them off, but the infantry was too tired to take up the chase.
Beauregard had asked for help. He needed Johnston's army in the Shenandoah Valley to join up with him to even up the odds against McDowell for the impending battle at Manassas. Confederate President Jefferson Davis ordered Johnston to move over to Manassas, and Johnston began on this day.
Johnston had to break away from Patterson's Union army in his front, but that was no problem. J. E. B. Stuart's cavalry was keeping Patterson busy, and Patterson, in violation of his vague orders to contain Johnston, had moved over to Charles Town instead of advancing directly on the Confederate position. Johnston moved his troops to the nearest railhead and began moving them to Manassas Junction.