The Federal army accordingly moved on the track of its adversary, and, on the afternoon of the same day (September 15th), found itself in sight of Lee's forces drawn up on the western side of Antietam Creek, near the village of Sharpsburg.
[Footnote 1: All along the march he had fired signal-guns to inform the officer in command at Harper's Ferry of his approach.] At last the great opponents were in face of each other, and a battle, it was obvious, could not long be delayed.
IV.
THE PRELUDE TO SHARPSBURG.
General Lee had once more sustained a serious check from the skill and soldiership of the officer who had conducted the successful retreat of the Federal army from the Chickahominy to James River.
The defeat and dispersion of the army of General Pope on the last day of August seemed to have opened Pennsylvania to the Confederates.

On the 15th of September, a fortnight afterward, General McClellan, at the head of a new army, raised in large measure by the magic of his name, had pursued the victorious Confederate, checked his further advance, and, forcing him to abandon his designs of invasion, brought him to bay a hundred miles from the capital.

This was generalship, it would seem, in the true acceptation of the term, and McClellan, harassed and hampered by the authorities, who looked but coldly upon him, could say, with Coriolanus, "Alone I did it." Lee was thus compelled to give up his movement in the direction of Pennsylvania, and concentrate his army to receive the assault of General McClellan.

Jackson, marching with his customary promptness, joined him with a portion of the detached force on the next day (September 16th), and almost immediately those thunders which prelude the great struggles of history began.
General Lee had drawn up his army on the high ground west of the Antietam, a narrow and winding stream which flows, through fields dotted with homesteads and clumps of fruit and forest trees, to the Potomac.

Longstreet's corps was posted on the right of the road from Sharpsburg to Boonsboro, his right flank guarded by the waters of the stream, which here bends westward; on the left of the Boonsboro road D.H.