The Battle of Gettysburg is fought July 1–3, 1863, in and around the town of Gettysburg, Pennsylvania, by Union and Confederate forces during the American Civil War.

The battle involves the largest number of casualties of the entire war and is often described as the war's turning point.

Union Maj. Gen. George Meade's Army of the Potomac defeats attacks by Confederate Gen. Robert E. Lee's Army of Northern Virginia, halting Lee's invasion of the North.

After his success at Chancellorsville in Virginia in May 1863, Lee leads his army through the Shenandoah Valley to begin his second invasion of the North—the Gettysburg Campaign.

With his army in high spirits, Lee intends to shift the focus of the summer campaign from war-ravaged northern Virginia and hopes to influence Northern politicians to give up their prosecution of the war by penetrating as far as Harrisburg, Pennsylvania, or even Philadelphia.

Prodded by President Abraham Lincoln, Maj. Gen. Joseph Hooker moves his army in pursuit, but is relieved of command just three days before the battle and replaced by Meade.

Elements of the two armies initially collide at Gettysburg on July 1, 1863, as Lee urgently concentrate his forces there, his objective being to engage the Union army and destroy it.

Low ridges to the northwest of town are defended initially by a Union cavalry division under Brig. Gen. John Buford, and soon reinforced with two corps of Union infantry.

However, two large Confederate corps assault  them from the northwest and north, collapsing the hastily developed Union lines, sending the defenders retreating through the streets of the town to the hills just to the south.

On the second day of battle, most of both armies have assembled.

The Union line is laid out in a defensive formation resembling a fishhook.

In the late afternoon of July 2, Lee launches a heavy assault on the Union left flank, and fierce fighting rages at Little Round Top, the Wheatfield, Devil's Den, and the Peach Orchard.

On the Union right, Confederate demonstrations escalate into full-scale assaults on Culp's Hill and Cemetery Hill.

All across the battlefield, despite significant losses, the Union defenders hold their lines.

On the third day of battle, fighting resumes on Culp's Hill, and cavalry battles rage to the east and south, but the main event is a dramatic infantry assault by twelve thousand five hundred Confederates against the center of the Union line on Cemetery Ridge, known as Pickett's Charge.

The charge is repulsed by Union rifle and artillery fire, at great loss to the Confederate army.

Lee leads his army on a torturous retreat back to Virginia.

Between forty-six thousand and fifty-one thousand soldiers from both armies are casualties in the three-day battle, the most costly in US history.

On November 19, President Lincoln uses the dedication ceremony for the Gettysburg National Cemetery to honor the fallen Union soldiers and redefine the purpose of the war in his historic Gettysburg Address.