l 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 h