<P> After two days of marching slowly in the sweltering heat, the Union army was allowed to rest in Centreville . McDowell reduced the size of his army to approximately 31,000 by dispatching Brig. Gen. Theodore Runyon with 5,000 troops to protect the army's rear . In the meantime, McDowell searched for a way to outflank Beauregard, who had drawn up his lines along Bull Run . On July 18, the Union commander sent a division under Brig. Gen. Daniel Tyler to pass on the Confederate right (southeast) flank. Tyler was drawn into a skirmish at Blackburn's Ford over Bull Run and made no headway . Also on the morning of 18 July Johnston had received a telegram suggesting he go to Beauregard's assistance if possible . Johnston marched out of Winchester about noon, while Stuart's cavalry screened the movement from Patterson . Patterson was completely deceived . One hour after Johnston's departure Patterson telegraphed Washington, "I have succeeded, in accordance with the wishes of the General - in - Chief, in keeping General Johnston's force at Winchester ." </P> <P> For the maneuver to be successful McDowell felt he needed to act quickly . He had already begun to hear rumors that Johnston had slipped out of the valley and was headed for Manassas Junction . If the rumors were true, McDowell might soon be facing 34,000 Confederates, instead of 22,000 . Another reason for quick action was McDowell's concern that the ninety - day enlistments of many of his regiments were about to expire . "In a few days I will lose many thousands of the best of this force", he wrote Washington on the eve of battle . In fact, the next morning two units of McDowell's command, their enlistments expiring that day, would turn a deaf ear to McDowell's appeal to stay a few days longer . Instead, to the sounds of battle, they would march back to Washington to be mustered out of service . </P> <P> Becoming more frustrated, McDowell resolved to attack the Confederate left (northwest) flank instead . He planned to attack with Brig. Gen. Daniel Tyler's division at the Stone Bridge on the Warrenton Turnpike and send the divisions of Brig . Gens . David Hunter and Samuel P. Heintzelman over Sudley Springs Ford . From here, these divisions could march into the Confederate rear . The brigade of Col. Israel B. Richardson (Tyler's Division) would harass the enemy at Blackburn's Ford, preventing them from thwarting the main attack . Patterson would tie down Johnston in the Shenandoah Valley so that reinforcements could not reach the area . Although McDowell had arrived at a theoretically sound plan, it had a number of flaws: it was one that required synchronized execution of troop movements and attacks, skills that had not been developed in the nascent army; it relied on actions by Patterson that he had already failed to take; finally, McDowell had delayed long enough that Johnston's Valley force was able to board trains at Piedmont Station and rush to Manassas Junction to reinforce Beauregard's men . </P> <P> On July 19--20, significant reinforcements bolstered the Confederate lines behind Bull Run . Johnston arrived with all of his army, except for the troops of Brig. Gen. Kirby Smith, who were still in transit . Most of the new arrivals were posted in the vicinity of Blackburn's Ford, and Beauregard's plan was to attack from there to the north toward Centreville . Johnston, the senior officer, approved the plan . If both of the armies had been able to execute their plans simultaneously, it would have resulted in a mutual counterclockwise movement as they attacked each other's left flank . </P>

Terrain of the first battle of bull run