Confederate army had crossed the Potomac.

The conduct of the battle on the left has given rise to several
criticisms, among which the most prominent has been that Porter's
corps, which lay in reserve, was not put in at the same time with
the Ninth Corps. It has been said that some of them were engaged or
in support of the cavalry and artillery at the centre. This does not
appear to have been so to any important extent, for no active
fighting was going on elsewhere after Franklin's corps relieved
Sumner's about noon. McClellan's reports do not urge this. He
answered the criticism by saying that he did not think it prudent to
divest the centre of all reserve troops. No doubt a single strong
division, marching beyond the left flank of the Ninth Corps, would
have so occupied A. P. Hill's division that our movement into
Sharpsburg could not have been checked, and, assisted by the advance
of Sumner and Franklin on the right, would apparently have made
certain the complete rout of Lee. As troops are put in reserve, not
to diminish the army, but to be used in a pinch, I am convinced that
McClellan's refusal to use them on the left was the result of his
rooted belief, through all the day after Sedgwick's defeat, that Lee
was overwhelmingly superior in force, and was preparing to return a
crushing blow upon our right flank. He was keeping something in hand
to fill a gap or cover a retreat, if that wing should be driven
back. Except in this way, also, I am at a loss to account for the
inaction of the right during the whole of our engagement on the
left. Looking at our part of the battle as only a strong diversion
to prevent or delay Lee's following up his success against Hooker
and the rest, it is intelligible. I certainly so understood it at
the time, as my report witnesses, and McClellan's original report
sustains this view. [Footnote: Official Records, vol. xix. pt. i.
pp. 31, 426.] If he had been impatient to have our attack delivered
earlier, he had reason for double impatience that Franklin's fresh
troops should assail Lee's left simultaneously with our assault of
his other wing, unless he regarded action there as hopeless, and
looked upon our movement as a sort of forlorn hope to keep Lee from
following up his advantages.

But even these are not all the troublesome questions requiring an
answer. It will be remembered that Franklin's corps, after forcing
Crampton's Gap, had remained in Pleasant Valley between Rohrersville