The attack at the bridge was actually unnecessary, as nearby fords would have allowed Burnside's troops to simply wade across the Antietam Creek. But, operating without knowledge of the fords, Burnside focused on the bridge, which was known locally as the "lower bridge," as it was the southernmost of several bridges crossing the creek.
On the western side of the creek, a brigade of Confederate soldiers from Georgia positioned themselves on bluffs overlooking the bridge. From this perfect defensive position the Georgians were able to hold off the Union assault on the bridge for hours.
A heroic charge by troops from New York and Pennsylvania finally took the bridge in the early afternoon. But once across the creek, Burnside hesitated and didn't press his attack forward.
Union Troops Advanced and Were Met By Confederate Reinforcements
By the end of the day, his troops had approached the town of Sharpsburg, and if they continued it was possible that Burnside's men could cut off Lee's line of retreat across the Potomac River into Virginia.
With amazing luck, part of Lee's army suddenly arrived on the field, having marched from their earlier action at Harper's Ferry. They managed to stop Burnside's advance.
As the day came to an end, the two armies faced each other across fields covered with thousands of dead and dying men. Many thousands of wounded were carried to makeshift field hospitals.
The casualties were stunning. It was estimated that 23,000 men had been killed or wounded that day at Antietam.
The following morning both armies skirmished slightly, but McClellan, with his usual caution, did not press the attack. That night Lee began evacuating his army, retreating across the Potomac River back into Virginia.