Nevertheless, the situation of the Confederates had become suddenly complicated.  When the march into Maryland was begun, three towns in the Valley were held by the Federals. 3000 infantry and artillery occupied Winchester. 3000 cavalry were at Martinsburg; and Harper’s Ferry, in process of conversion into an intrenched camp, had a garrison of 8000 men.  Lee was well aware of the presence of these forces when he resolved to cross the Potomac, but he believed that immediately his advance threatened to separate them from the main army, and to leave them isolated, they would be ordered to insure their safety by a timely retreat.  Had it depended upon McClellan this would have been done.  Halleck, however, thought otherwise; and the officer commanding at Harper’s Ferry was ordered to hold his works until McClellan should open communication with him.

On arrival at Frederick, therefore, the Confederates, contrary to anticipation, found 14,000 Federals still established in their rear, and although Winchester had been evacuated,* (* On the night of September 2.  Lee’s Report, O.R. volume 19 part 1 page 139.) it was clear that Harper’s Ferry was to be defended.  The existence of the intrenched camp was a serious obstacle to the full development of Lee’s designs.  His line of communication had hitherto run from Rapidan Station to Manassas Junction, and thence by Leesburg and Point of Rocks to Frederick.  This line was within easy reach of Washington, and liable to be cut at any moment by the enemy’s cavalry.  Arrangements had therefore been already made to transfer the line to the Valley.  There, sheltered by the Blue Ridge, the convoys of sick and wounded, of arms, clothing, and ammunition, could move in security from Staunton to Shepherdstown, and the recruits which were accumulating at Richmond be sent to join the army in Northern territory.  But so long as Harper’s Ferry was strongly garrisoned this new line would be liable to constant disturbance, and it was necessary that the post should either be masked by a superior force, or carried by a coup de main.  The first of these alternatives was at once rejected, for the Confederate numbers were too small to permit any permanent detachment of a considerable force, and without hesitation Lee determined to adopt the bolder course. 25,000 men, he considered, would be no more than sufficient to effect his object.  But 25,000 men were practically half the army, and the plan, when laid before the generals, was not accepted without remonstrance.  Longstreet, indeed, went so far as to refuse command of the detachment.  “I objected,” he writes, “and urged that our troops were worn with marching and were on short rations, and that it would be a bad idea to divide our forces while we were in the enemy’s country, where he could get information, in six or eight hours, of any movement we might make.  The Federal army, though beaten at the Second Manassas, was not disorganised, and it would certainly come out