The Antietam, a half-mile below Burnside's bridge, makes a sudden curve toward the west. It is crossed by one other bridge, at Antietam Iron-works, and then joins the Potomac. By throwing General Burnside across the Antietam, General McClellan designed not to turn the right of Lee and gain possession of his only line of retreat to Shepardstown, but to carry the heights, then pass along the crest towards the right.[73] But this movement isolated General Burnside from the army. He must hold the bridge or be cut off. He would be in a cul de sac, a bag with only one place of escape, at the Antietam Iron-works.
When General Lee saw the preparations of Burnside to advance, after having carried the bridge, he weakened his left to strengthen his right. Hood, who was lying in reserve behind Jackson, was sent down. Longstreet moved some of his brigades. Jackson made a demonstration at Poffenberger's, already noticed, to make McClellan fear an attack at that point.
General Lee intended to do more than merely hold his line against Burnside.[74] By massing his troops at Sharpsburg, when Burnside was far enough advanced, Lee intended to seize the bridge and cut off Burnside's retreat.
Burnside's divisions crossed the stream at the bridge and at the ford, and formed for an advance upon the heights near the town. Wilcox was on the right, supported by Rodman in the center, Scammon's brigade on the left, and Sturgis in rear of Rodman.
While the troops were crossing and forming, Longstreet's and A. P. Hill's batteries kept up a constant fire of shells. Clark's, Durrell's, Cook's, and Simmons's batteries went across the bridge, gained the crest of the hill beyond, came into position, and opened fire in reply.
General Wilcox was on the road leading from the bridge to Sharpsburg, which passes up a ravine. A brook which has its rise beyond the town, gurgles by the roadside. Rebel batteries on the hills in front of the town enfiladed the ravine, sweeping it from the town to the river. There was no shelter for the troops while advancing. They must take the storm in their faces.
Neither was there any cover for Rodman, Sturgis, and Scammon. The ground, from the stone wall on the top of the river bank to the hills occupied by Hill and Longstreet, was all tillage land,—wheat-fields, and pastures, and patches of corn. There were fences to throw down, hills to climb, all to be done under fire from cannon arranged in crescent form, pouring down a concentrated fire from the heights.
The signal officer, upon Elk Ridge, five hundred feet above the battle-field, beholds all the operations of the Rebel army. From his lookout, with his telescope, he can sweep the entire field. His assistant waves a flag, and an officer, with his eye at the telescope by McClellan's head-quarters, reads a message of this import, transmitted by the little flag.
"The Rebels are weakening their left, and concentrating their troops upon their right."
The officer writes it in his message book, tears out the leaf, and hands it to General McClellan. He thus knows Lee's movements, the disposition of his forces, as well as if he himself had looked from the mountain summit upon the moving column.