It was Longstreet's actions at the Battle of Gettysburg, fought in Pennsylvania in July 1863, that haunted him after the war. That battle marked the first major campaign for the army without Jackson, who died in May 1863, and the beginning of problems within the army's high command.

Prior to the summer of 1863, Longstreet offered a plan to the Richmond government designed to relieve pressure on Vicksburg. His proposal was not adopted in favor of Lee's plan to invade the North. Lee's plan was designed to relieve Virginia of Union troops, giving farmers time to bring in their crops; to threaten Northern cities, convincing the Union government that a continued war was useless; and to relieve other parts of the Confederacy by causing Union armies in the west to move east. 

Longstreet told Lee that offensive attacks on the Federal position along Cemetery Ridge were doomed to failure. He preferred to flank the Union line and establish a defensive position somewhere between the Union Army and Washington DC. He felt that Meade would then be forced to attack a well-established Confederate line. 

Lee's refusal to fight defensively rankled Longstreet, who barely concealed his displeasure. Still, his assault in the afternoon on July 2 virtually destroyed the Union Army's III Corps, but failed to capture the prominent Round Tops that dominated the Union position. Lee refused to relinquish the initiative, however, and issued plans for a massive frontal assault on the Union center the following day. 

With the shells screaming and exploding all around him, he was observed by Brigadier General J. L. Kemper of Pickett's division: 

Longstreet rode slowly and alone immediately in front of our entire line. He sat his large charger with a magnificent grace and composure I never before beheld. His bearing was to me the grandest moral spectacle of the war. I expected to see him fall every instant. Still he moved on, slowly and majestically, with an inspiring confidence, composure, self-possession, and repressed power in every movement and look, that fascinated me.