In the Chapter 5, July 3rd, it says, "Then he [Longstreet] turned and went out into the field to say goodbye..." To whom or what is Longstreet saying goodbye?
It is not completely clear to whom Longstreet is saying goodbye. It seems, however, that he is saying goodbye to the Confederate dead left on the field. Longstreet has just met with Robert E. Lee, who has admitted that his decision to order Pickett's Charge was a major error. He announces that they will begin to withdraw from the field, and that is why Longstreet is saying goodbye. Immediately before encountering Lee, Longstreet had been watching the burial parties beginning to collect and inter the dead from the battle:
He looked out at the burial parties and the lights beginning to come on across the field like carrion fireflies. All that was left now was more dying. It was final defeat. They had all died and it had accomplished nothing, the wall was unbroken, the blue line was sound.
It seems that after Lee rode off, Longstreet was going into the field to bid goodbye to the dead before leading the retreat out of Pennsylvania. He is disconsolate and sees little point in continuing to fight and die.