Gettysburg, Battle of, battle fought July 1 through July 3, 1863, considered by most military historians the turning point in the American Civil War. The Battle of Gettysburg was a decisive engagement in that it arrested the Confederates' second and last major invasion of the North, destroyed their offensive strategy, and forced them to fight a defensive war in which the inadequacies of their manufacturing capacity and transportation facilities doomed them to defeat.

The Army of the Potomac, under the Union general George Gordon Meade, numbered about 85,000; the Confederate army, under General Robert E. Lee, numbered about 75,000. After the Battle of Chancellorsville on May 2 to 4, an important victory for the Confederates, Lee divided his army into three corps, commanded by three lieutenant generals: James Longstreet, Richard Stoddert Ewell, and Ambrose Powell Hill. Lee then formulated a plan for invading Pennsylvania, hoping to avert another federal offensive in Virginia and planning to fight if he could get the federal army into a vulnerable position; he also hoped that the invasion might increase Northern war-weariness and lead the North to recognize the independence of the Confederate States of America. In pursuit of this plan, Lee crossed the Blue Ridge Mountains, proceeded up the Shenandoah Valley, and, crossing Maryland, entered Pennsylvania. Upon learning federal troops were north of the Potomac, Lee decided to concentrate his whole army at Gettysburg.

On June 30, Confederate troops from General Hill's corps, on their way to Gettysburg, noted federal troops that Meade had moved down to intercept the Confederate army. The battle began on July 1 outside of Gettysburg with an encounter between Hill's advance brigades and the federal cavalry division commanded by Major General John Buford, supported by infantry under Major General John Fulton Reynolds. Hill encountered stubborn resistance, and the fighting was inconclusive until Ewell arrived from the north in the afternoon. The Confederates pushed against General Oliver Howard's corps and forced the federal troops to retire from their forward positions to Culp's Hill and Cemetery Ridge, southeast of Gettysburg. The fighting had been heavy on both sides, but the Union troops suffered more losses. More than 4000 men were taken prisoner by the Confederates, and Federal General John Reynolds was killed in battle. The federals did manage to capture Confederate General Archer, the first Confederate officer to be taken prisoner after Lee assumed command of the Confederate army. The corps led by Ewell did not move in to attack the Union troops but waited for General Longstreet to bring in his corps to reinforce the outnumbered Confederate troops.

On the following day, July 2, Meade formed his main forces in the shape of a J (resembling a fishhook), extending westward from Culp's Hill and southward along Cemetery Ridge to the hills of Little Round Top and Round Top. The Confederates, on the other hand, were deployed in a long, thin, concave line, with Longstreet and Ewell on the flanks and Hill in the center.