Why they Fight

From Wikipedia:

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 bloodiest[1] battle of the American Civil War and is frequently cited as the war's turning point. Union Major General George G. Meade's Army of the Potomac decisively defeated attacks by Confederate General Robert E. Lee's Army of Northern Virginia, ending Lee's second and final invasion of the North.

Following his brilliant 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 the 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 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. Over 51,000 Americans were casualties in the three-day battle, men commemorated by President Lincoln that November in his historic Gettysburg Address.