The accidental battle at Gettysburg inflicted at least 46,000 casualties — split roughly evenly between the two sides. Almost 8,000 men were killed in the three days of the battle. The North's casualties were appalling, but the South lost a greater percentage of its strength. On the night of July 3, Lee ordered a withdrawal to Virginia.

In the days that followed, Lee's campaign of invasion turned into a desperate retreat across Maryland. The armies fought a series of smaller battles over the next week or so — at Boonsboro, Funkstown and Williamsport.

Four months later, Lincoln traveled to the battlefield, where he delivered a speech that did not go down in history as the Taneytown Address.

Contrary to popular belief, Gettysburg did not seal the fate of the Confederacy, Goodheart said. Nor was it the only crushing blow to the South that week.

"The fall of Vicksburg [on July 4] was actually a more unequivocal and absolute Union victory than Gettysburg," he said. Even then, Goodheart added, the outcome of the war would remain in doubt for more than a year. It would take the September 1864 fall of Atlanta — ensuring Lincoln's re-election — to make a Union victory inevitable.

Lee continued to fight until April 1865, when he was trapped at Appomattox, Va., and surrendered. After the war, admirers sought to absolve him of blame for the Gettysburg defeat and to put the onus on various subordinates. Modern historians haven't been so kind, noting that the fateful decision to order Pickett's Charge was Lee's alone.

After an initial burst of acclaim in the North, Meade's reputation faded as the public learned more about the pivotal role of able subordinates. According to Guelzo, a big difference in the battle was that Meade's supporting cast — generals such as Winfield Scott Hancock and Gouverneur Warren — showed initiative while Lee's did not.

"Here's a case in which the great military leader [Lee] finds his greatness boomeranging against him. He was so great that everybody was looking at him to make the great decisions," Guelzo said.

While Civil War historians have not been kind to the irritable, overcautious and moody Meade, nobody could erase the fact that Gettysburg was a pivotal victory achieved while he was in charge. It is perhaps fitting that the most populous Army base in the state where he wanted to fight the biggest battle of his life is Fort George G. Meade.

"In the final accounting, Gettysburg was a victory, for George Meade, for the Army of the Potomac, and for the Union," Guelzo writes. It was, he adds, "decisive enough that after Gettysburg, the sun never shone for the South again."

More on Gettysburg

Gettysburg anniversary events:

Historic battle photos:

National Park Service photo gallery:

Gettysburg battle timeline:

Gettysburg campaign timeline:

Maryland Civil War Trails:

U.S. Army history of Gettysburg campaign:

U.S. Army interactive battle map:

Events at Gettysburg national park

•Living History Camps: Union and Confederate re-enactors will present demonstrations near the Pennsylvania Memorial and at Pitzer's Woods on Monday through Wednesday.

•Key Moment Programs: On Monday through Thursday, park rangers will present programs at locations where key events occurred.

•Hikes: From Monday through Thursday, park rangers will lead hourlong hikes on phases of the battle and its aftermath.

•Battlefield Experience Programs: Park rangers will provide programs on critical moments at the approximate time they occurred 150 years ago — including a march Wednesday across the field of Pickett's Charge.

The Pipe Creek Line: An Overview: