Pope being held in check by the main army in front, General Jackson was sent around Pope's right wing to flank him. Passing through Thoroughfare Gap he reached the railroad at Bristoe's Station, in the rear of Pope's army (August 26). General Pope, seeing an opportunity while Lee's army was thus divided to cut it up in detail, turned upon Jackson. But the Army of the Potomac not promptly reinforcing him, his plans failed, and instead of "bagging " Jackson's division, he was compelled, with only forty thousand men, to fight the entire Confederate army on the old battlefield of Bull Run. Exhausted, cut off from supplies, and overwhelmed by numbers, the shattered remains of the Union forces were glad to take refuge within the fortifications of Washington. [Footnote: During the pursuit by Lee's forces, an engagement took place at _Chantilly_ (September 1). It cost the Union army two able officers--Generals Stevens and Kearney. The latter, especially, was devotedly loved by his soldiers. On the battlefield, brandishing his sword in his only hand, and taking the reins in his teeth, he had often led them in the most desperate and irresistible charges.] _The Effect_.--In this brief campaign the Union army lost thirty thousand men and vast supplies, while the way to Washington was opened to the Confederates. The Capital had not been in such peril since the war began. Without, was a victorious army; within, were broken battalions and no general. INVASION OF MARYLAND.--Flushed with success, Lee now crossed the Potomac and entered Maryland, hoping to secure volunteers and incite an insurrection. [Footnote: This was Sept. 5, the very day that Bragg entered Kentucky on his great raid.] McClellan, who had been restored to the command of the Army of the Potomac, reorganized the shapeless mass and set out in pursuit. On the way he found a copy of Lee's order of march. Learning from this that Lee had divided his forces, and that but a portion remained in his front, he hastened in pursuit. [Footnote: Lee had sent Jackson with twenty-five thousand men against _Harper's Ferry_. That redoubtable leader quickly carried the heights which overlook the village, forced Colonel Miles, with eleven thousand men, to surrender, and then hastened back to take part in the approaching contest.] Overtaking the Confederate rear at _South Mountain_, and forcing the passes, the Union army poured into the valley beyond (map opp. p. 223).