him back across the Potomac.  Nothing but sheer necessity justified the advance of the Army of the Potomac to South Mountain and Antietam in its then condition.  The purpose of advancing from Washington was simply to meet the necessities of the moment by frustrating Lee’s invasion of the Northern States, and when that was accomplished, to push with the utmost rapidity the work of reorganisation and supply, so that a new campaign might be promptly inaugurated with the army in condition to prosecute it to a successful termination without intermission."* (* Battles and Leaders volume 2 page 554.)

And in his official report, showing what the result of a Confederate success might well have been, he says:  “One battle lost and almost all would have been lost.  Lee’s army might have marched as it pleased on Washington, Baltimore, Philadelphia, or New York.  It could have levied its supplies from a fertile and undevastated country, extorted tribute from wealthy and populous cities, and nowhere east of the Alleghanies was there another organised force to avert its march."* (* O.R. volume 19 part 1 page 65.)

3.  The situation in the West was such that even a victory in Maryland was exceedingly desirable.  Confederate movements in Tennessee and Kentucky had won a measure of success which bade fair to open up a brilliant opportunity.  Should the Federals be defeated in both the theatres of war, the blow would be felt throughout the length and breadth of the Northern States; and, in any case, it was of the utmost importance that all McClellan’s troops should be retained in the East.

So, when the tidings came of Jackson’s victory at Harper’s Ferry, both armies braced themselves for the coming battle, the Confederates in the hope that it would be decisive of the war, the Federals that it would save the capital.  But the Confederates had still a most critical time before them, and Lee’s daring was never more amply illustrated than when he made up his mind to fight on the Antietam.  McClellan’s great army was streaming through the passes of the South Mountain.  At Rohrersville, six miles east of the Confederate bivouacs, where he had halted as soon as the cannonade at Harper’s Ferry ceased, Franklin was still posted with 20,000 men.  From their battle-field at Turner’s Gap, ten miles from Sharpsburg, came the 70,000 which composed the right and centre; and on the banks of the Antietam but 15,000 Southerners were in position.  Jackson had to get rid of his prisoners, to march seventeen miles, and to ford the Potomac before he could reach the ground.  Walker was twenty miles distant, beyond the Shenandoah; and McLaws, who would be compelled by Franklin’s presence near Rohrersville to cross at Harper’s Ferry and follow Jackson, over five-and-twenty.  Would they be up before McClellan attacked?  Lee, relying on McClellan’s caution and Jackson’s energy, answered the question in the affirmative.