"I figured this would probably be the only time I'd ever see something like this in my life," said Richard Garvin, of Hagerstown, between cannon blasts.
He and other spectators said they were impressed by the roar of the mock firepower, fired in volley after volley by more than 11,000 re-enactors wearing blue and gray.
The re-enactors moved in lines, shoulder to shoulder, as they entered the battle in groups of hundreds. Bugles were blown, and state and regimental flags were waved. Officers barked orders and Confederate soldiers cried out the famous rebel yell.
"Look how real it is. There's no plastic. Everything is pretty real," said Charles Botwright of Crofton, Md.
Blasting caps planted in the ground to create "explosions" started a small brush fire on the farmland used for the battlefield. The blaze was extinguished by fire crews on the site.
"Small problem," event coordinator Greg Larsen said of the fire.
But on the afternoon of Sept. 17, 1862, Lee was having serious problems. After the fierce fighting that day in places like the Cornfield and Sunken Road, Union forces under the command of Maj. Gen. Ambrose Burnside finally, at 1 p.m., forced their way across the bridge that later would bear his name.
The Union Army was on the move and the Confederates were in trouble. Lee's forces were backed up against the Potomac River and the only reserve division he had - A.P. Hill's - had started the day in Harpers Ferry, in what then was Virginia.
"This (A.P. Hill's assault) is very important because the whole Confederate Army could have been overrun that day," said Tom Stanton, a Confederate artillery soldier from Rochester, N.Y.
But Union commander Gen. George B. McClellan - never known to take risks on the battlefield - missed his chance to annihilate Lee's forces by refusing to order additional troops to support a final attack.
Patrick Falci, a re-enactor/historian who portrayed Hill at the re-enactment described the scene: "It's a very desperate situation. McClellan basically has Robert E. Lee on the run and it's up to A.P. Hill to save the day."
He did. At about 3:30 p.m. Hill's men arrived from Harpers Ferry, hit the left flank of Burnside's corps south of Sharpsburg, and stopped the Union advance cold. By 5:30 p.m., the battle and the bloodiest day in the country's history were over.
"A.P. Hill came in just at the right time," John said.
Many historians and authors believe it was the combination of Hill's assault and McClellan's trademark caution that spared the Confederates a disastrous defeat and prevented the Union from scoring a major victory.
"There is a definite `what if?' element about this particular engagement," John said.
The war would go on for 2 1/2 more bloody years.