In The Killer Angels, General James Longstreet is well aware of the causes of his fellow Confederates. He knows that some, like the late Stonewall Jackson, saw the Southern cause as a holy crusade for independence. Others see it as a fight to protect and preserve the institution of slavery. Longstreet does not much concern himself with either of these causes.
The war was about slavery, all right. That was not why Longstreet fought but that was what the war was about, and there was no point in talking about it, never had been. (Part 3, Chapter 5)
Indeed, by the time of the Battle of Gettysburg, he believes that these causes have already been lost.
For Longstreet, his personal "cause" is simply ending the war. He is weary and tired of the many privations forced upon him by the conflict. Also, the sudden deaths of three of his children have thoroughly demoralized the general. He no longer has the same enthusiasm and esprit de corps that he was once known for. He has seen too much senseless bloodshed and death, and now he just wants to bring the war to an end so that he can go home.