The Battle of Gettysburg (July 1 – July 3, 1863), fought in, and around the town of Gettysburg, Pennsylvania, as part of the Gettysburg Campaign, was the battle with the largest number of casualties in the American Civil War Union Maj. Gen. George Gordon Meade's Army of the Potomac defeated attacks by Confederate General Robert E. Lee's Army of Northern Virginia, ending Lee's invasion of the North.
Following his success at Chancellorsville in May 1863, Lee led his army through the Shenandoah Valley for his second invasion of the North, hoping to reach as far as Harrisburg, Pennsylvania, or even Philadelphia, and to influence Northern politicians to give up their prosecution of the war. Prodded by President Abraham Lincoln, Maj. Gen. Joseph Hooker moved his army in pursuit but was relieved almost on the eve of battle and replaced by Meade.
The two armies began to collide at Gettysburg on July 1, 1863, as Lee urgently concentrated his forces there. Low ridges to the northwest of town were defended initially by a Union cavalry division, which was soon reinforced with two corps of Union infantry. However, two large Confederate corps assaulted them from the northwest and north, collapsing the hastily developed Union lines, sending the defenders retreating through the streets of town to the hills just to the south.
On the second day of battle, most of both armies had assembled. The Union line was laid out in a defensive formation resembling a fishhook. Lee launched a heavy assault on the Union left flank, and fierce fighting raged at Little Round Top, the Wheatfield, Devil's Den, and the Peach Orchard. On the Union right, demonstrations escalated into full-scale assaults on Culp's Hill and Cemetery Hill. Across the battlefield, despite significant losses, the Union defenders held their lines.
On the third day of battle, July 3, fighting resumed on Culp's Hill, and cavalry battles raged to the east and south, but the main event was a dramatic infantry assault by 12,500 Confederates against the center of the Union line on Cemetery Ridge. Pickett's Charge was repulsed by Union rifle and artillery fire at great losses to the Confederate army. Lee led his army on a torturous retreat back to Virginia. Between 46,000 and 51,000 Americans were casualties in the three-day battle. That November, President Lincoln used the dedication ceremony for the Gettysburg National Cemetery to honor the fallen and redefine the purpose of the war in his historic Gettysburg Address.

Background and movement to battle
Further information: Battle of Gettysburg, First Day
General Buford realized the importance of the high ground directly to the south of Gettysburg, knowing that if the Confederates could gain control of the heights, Meade's army would have difficulty dislodging them. He decided to utilize three ridges west of Gettysburg: Herr Ridge, McPherson Ridge, and Seminary Ridge (proceeding west to east toward the town). These were appropriate terrain for a delaying action by his small division against superior Confederate infantry forces, meant to buy time awaiting the arrival of Union infantrymen who could occupy the strong defensive positions south of town at Cemetery Hill, Cemetery Ridge, and Culp's Hill.

First day of battle
Further information: Second Day, Little Round Top, Culp's Hill, and Cemetery Hill

Second day of battle
Throughout the evening of July 1 and morning of July 2, most of the remaining infantry of both armies arrived on the field, including the Union II, III, V, VI, and XII Corps. Longstreet's third division, commanded by George Pickett, had begun the march from Chambersburg early in the morning; it did not arrive until late on July 2.

Plans and movement to battle
As Longstreet's divisions slammed into the Union III Corps, Meade had to send 20,000 reinforcements

Attacks on the Union left flank
About 7:00 p.m., the Second Corps' attack by Johnson's division on Culp's Hill got off to a late start. Most of the hill's defenders, the Union XII Corps, had been sent to the left to defend against Longstreet's attacks, and the only portion of the corps remaining on the hill was a brigade of New Yorkers under Brig. Gen. George S. Greene. Because of Greene's insistence on constructing strong defensive works, and with reinforcements from the I and XI Corps, Greene's men held off the Confederate attackers, although the Southerners did capture a portion of the abandoned Federal works on the lower part of Culp's Hill.

Attacks on the Union right flank
Further information: Culp's Hill, Pickett's Charge, and Third Day cavalry battles
General Lee wished to renew the attack on Friday, July 3, using the same basic plan as the previous day: Longstreet would attack the Federal left, while Ewell attacked Culp's Hill.

Third day of battle

The Confederate retreat
The news of the Union victory electrified the North. A headline in The Philadelphia Inquirer was "VICTORY! WATERLOO ECLIPSED!" New York diarist George Templeton Strong wrote:
The results of this victory are priceless. ... The charm of Robert E. Lee's invincibility is broken. The Army of the Potomac has at last found a general that can handle it, and has stood nobly up to its terrible work in spite of its long disheartening list of hard-fought failures. ... Copperheads are palsied and dumb for the moment at least. ... Government is strengthened four-fold at home and abroad.

Reaction to the news of the Union victory
The Confederates had lost politically as well as militarily. During the final hours of the battle, Confederate Vice President Alexander Stephens was approaching the Union lines at Norfolk, Virginia, under a flag of truce. Although his formal instructions from Confederate President Jefferson Davis had limited his powers to negotiations on prisoner exchanges and other procedural matters, historian James M. McPherson speculates that he had informal goals of presenting peace overtures. Davis had hoped that Stephens would reach Washington from the south while Lee's victorious army was marching toward it from the north. President Lincoln, upon hearing of the Gettysburg results, refused Stephens's request to pass through the lines. Furthermore, when the news reached London, any lingering hopes of European recognition of the Confederacy were finally abandoned. Henry Adams wrote, "The disasters of the rebels are unredeemed by even any hope of success. It is now conceded that all idea of intervention is at an end."

Impact on the Confederacy
The armies would move on, but Gettysburg had much cleaning up to do. The two armies had suffered between 46,000 and 51,000 casualties. Union casualties were 23,055 (3,155 killed, 14,531 wounded, 5,369 captured or missing). were burned in a series of piles south of town; townsfolk became violently ill from the stench. Pennsylvania and New York state militia patrolled the Gettysburg battlefield and secured as much of the remaining military property as possible, often arresting souvenir hunters and forcing them to assist in the disposal of the dead horses. The ravages of war would still be evident in Gettysburg more than four months later when, on November 19, the Soldiers' National Cemetery was dedicated. During this ceremony, President Abraham Lincoln with his Gettysburg Address re-dedicated the Union to the war effort.
Today, the Gettysburg National Cemetery and Gettysburg National Military Park are maintained by the U.S. National Park Service as two of the nation's most revered historical landmarks.

Casualties

Historical assessment
Throughout the campaign, General Lee seemed to have entertained the belief that his men were invincible; most of Lee's experiences with the army had convinced him of this, including the great victory at Chancellorsville in early May and the rout of the Federals at Gettysburg on July 1.

Assessment of Lee's leadership style
Gettysburg Campaign: Campaign Overview • Brandy Station • Second Winchester • Aldie • Middleburg • Upperville • Sporting Hill • Hanover • Carlisle • Gettysburg • Hunterstown • Fairfield • Williamsport • Boonsboro • Manassas Gap Actions at Gettysburg: First Day • Second Day • Third Day cavalry battles • Big Round Top • Cemetery Hill • Culp's Hill • Devil's Den • Little Round Top • Peach Orchard • Pickett's Charge • Wheatfield
American Civil War • Confederate Order of Battle • Union Order of Battle • Department of the Monongahela • Department of the Susquehanna • Gettysburg Address • Gettysburg National Cemetery • Cemetery Ridge • Seminary Ridge Army of the Potomac: I Corps • II Corps • III Corps • V Corps • VI Corps • XI Corps • XII Corps • 1st Minnesota • 20th Maine Army of Northern Virginia: First Corps • Second Corps • Third Corps • Cavalry Corps Gettysburg today: Battlefield • Town • College • Lutheran Theological Seminary Popular media: Gettysburg (film) • Gettysburg (game) • Gettysburg (music) • Cyclorama • Sid Meier's Gettysburg! • Terrible Swift Sword • The Killer Angels
Gettysburg Textbooks from Wikibooks • Gettysburg Images and media from Commons • Gettysburg News stories from Wikinews

Notes

Gottfried, Bradley M., The Maps of Gettysburg: An Atlas of the Gettysburg Campaign, June 3 – June 13, 1863, Savas Beatie, 2007, ISBN 978-1-932714-30-2.
Huntington, Tom, Pennsylvania Civil War Trails: The Guide to Battle Sites, Monuments, Museums and Towns, Stackpole books, 2007, ISBN 978-0-8117-3379-3.
Shaara, Michael, The Killer Angels: A Novel, David McKay Co., 1974, (reprinted by Ballantine Books, 2001), ISBN 978-0345444127.
Stackpole, Gen. Edward J., "They Met at Gettysburg", Stackpole Books, 1956, ISBN 0-8117-2089-6.