By late morning the fighting slid south to the center of Lee’s line, the wing commanded by Major General James Longstreet (South), where Confederate soldiers had taken cover in a worn-down farm lane. Years of wagon traffic had sunk the road below the level of the surrounding fields, turning an ordinary country path into a ready-made trench. Wave after wave of Union troops marched up the rising ground toward it and were shot down by the men sheltered in the sunken road. For hours the lane held.

One of those waves has a name. Among the Union brigades sent up the rising ground was the Irish Brigade, men recruited largely from Irish immigrant neighborhoods of New York and Boston, fighting under green regimental flags, led by Brigadier General Thomas Francis Meagher (North). Before they stepped off toward the lane, their chaplain, Father William Corby, rode the length of the line on horseback and gave the men general absolution, a Catholic priest’s blessing forgiving their sins all at once, the last rites of their faith granted in advance to soldiers about to walk into fire. Then they went forward. They walked into the worst of the sunken-road fire and were torn apart, losing on the order of 540 men in the attack, roughly three of every five who started up the slope. The green flags went down, were picked up, and went down again.

Then Union soldiers worked around to a spot where they could fire straight down the length of the road, and the trench that had protected the Confederates became a killing chute with no cover. Men fell in heaps along its length. When it was over, the lane had a new name that it still carries: Bloody Lane. The Federals had finally punched a hole clean through the center of Lee’s army.

This was the moment. Lee’s center was broken open, his army split, his whole position one hard shove from collapse. Major General George B. McClellan had an entire fresh corps in reserve, Major General Fitz John Porter’s (North) V Corps, standing ready behind the lines, the men who could have driven through the gap and finished the Army of Northern Virginia for good. McClellan, certain as always that Lee had hidden masses of men somewhere, refused to send them in. The hole closed. The chance passed. It was the great missed chance of the day, the bigger army holding back the blow that could have ended its enemy.

At the southern end of the field, a third battle had been grinding on. Major General Ambrose Burnside (North) and his IX Corps spent much of the afternoon trying to cross a narrow stone bridge over Antietam Creek, defended by only a few hundred Georgia riflemen on the high bank above. The numbers were lopsided: thousands of men in blue against a few hundred in gray, hours of dying for a bridge they did not have to take, since the creek could be waded across at other spots not far away. The Georgians, dug in on the heights, picked off anyone who set foot on the span. It has been called Burnside’s Bridge ever since.

When Burnside’s men finally got across and reorganized, they pushed uphill toward Sharpsburg itself, into the rear of Lee’s exhausted, outnumbered army. Lee had spent his whole army to hold the morning’s fights. He had nothing behind his right but tired men and empty road, no fresh brigade left, while Burnside’s blue mass climbed toward the town that anchored his entire line. One more push might have finished the Army of Northern Virginia.

Then, out of the south, came the dust of marching men. They were Major General A. P. Hill’s (South) “Light Division,” and they had spent the day on the road, force-marching about 17 miles (27 km) from Harpers Ferry to reach the battlefield, some of them dressed in Union blue uniforms captured at the garrison they had just taken. They arrived at the worst possible moment for Burnside and the best for Lee, slammed into the exposed Union flank, and drove Burnside’s corps back down the slope they had just climbed. The line held. The Confederate army was still alive when the sun went down. The battle ended in the dark, with both armies right where they had nearly destroyed each other.