lellan had hastened his advance on the 13th, the passes of South
Mountain at Turner's and Fox's gaps would not have been occupied in
force by the enemy, and the condition of things would have been what
he believed it was on the morning of the 14th, when a single brigade
had been thought enough to support Pleasonton's reconnoissance.
Twenty-four hours had changed all that.

The turnpike from Boonsboro to Sharpsburg continues southward a
couple of miles, crossing the Potomac to Shepherdstown, which lies
on the Virginia side of the river. A bridge which formerly carried
the road over the stream had been burned; but not far below the
ruined piers was a ford, which was a pretty good one in the present
stage of water. Shepherdstown was the natural place of junction for
Lee and Jackson; but for Lee to have marched there at once would
have exposed Jackson to attack from the northern side of the
Potomac. The precious stores and supplies captured at Harper's Ferry
must be got to a place of safety, and this was likely to delay
Jackson a day or two. Lee therefore ordered McLaws to obstruct
Franklin's movement as much as he could, whilst he himself
concentrated the rest of Longstreet's corps at Sharpsburg, behind
the Antietam. If McClellan's force should prove overwhelming, the
past experience of the Confederate general encouraged him to believe
that our advance would not be so enterprising that he could not make
a safe retreat into Virginia. He resolved therefore to halt at
Sharpsburg, which offered an excellent field for a defensive battle,
leaving himself free to resume his aggressive campaign or to retreat
into Virginia according to the result.

McClellan had ordered Richardson's division of the Second Corps to
support the cavalry in the advance, and Hooker's corps followed
Richardson. [Footnote: Hooker's Report, Official Records, vol. xix.
pt. i. p. 216.] It would seem most natural that the whole of
Sumner's wing should take the advance on the 15th, though the
breaking up of organizations was so much a habit with McClellan that
perhaps it should not be surprising that one of Sumner's divisions
was thus separated from the rest, and that Burnside's right wing was
also divided. [Footnote: We must not forget the fact, however, that
the order dividing the army into wings was suspended on that
morning, and that this gives to the incident the air of an
intentional reduction of the wing commanders to the control of a
single corps. O