He had grown up believing in America and the individual and it was a stronger faith than his faith in God. This was the land where no man had to bow. In this place at last a man could stand up free of the past, free of tradition and blood ties and the curse of royalty and become what he wished to become. This was the first place on earth where the man mattered more than the state. (1.2.119)
Chamberlain's reflections here were more characteristic of the Northern than the Southern perspective. The South still had an aristocracy that practically seemed like royalty: slaves obviously couldn't rise to a higher social status, and poor Southern whites couldn't really do it, either. Although the North had its own issues, it's true that it was much easier for any free person to advance and make a living in there than in the South.
"You must tell them, and make it plain, that what we are fighting for is our freedom from the rule of what is to us a foreign government. That's all we want and that's what this war is all about. We established this country in the first place with strong state governments just for that reason, to avoid a central tyranny—" (1.4.170)
The Confederates claimed that the war was about states' rights: they didn't want the bigger, more populous North dictating what they should do in the South. Of course, the states' rights in question were specifically the ones that allowed slavery, but the Confederates sort of yadda-yadda-ed through that part. Back then, people identified more strongly with their states than many do today. The United States wasn't that old yet, and it was actually still forming, so the sense of all being a part of one big nation was less intense at the time. The Civil War changed that for good—it made the nation more important than the individual state.
"I must say, there are times when I'm troubled. But… couldn't fight against home. Not against your own family. And yet… we broke the vow." (3.3.104)
Longstreet admits that he and Lee had a vow of loyalty to the Union that they've broken; in fact, they're firing against troops marching under the very flag they'd sworn to defend. As it turned out, their loyalties to their states out-weighed that vow. Longstreet still doesn't seem totally comfortable with the whole thing, and (fun fact) after the war, he would become a Republican politician supporting Ulysses S. Grant's administration… even though Grant was the leader of the Union Army at the end of the war.
And so he took up arms willfully, knowingly, in perhaps the wrong cause against his own sacred oath and stood now upon alien ground he had once sworn to defend, sworn in honor, and he had arrived there really in the hands of God, without any choice at all; there had never been an alternative except to run away, and he could not do that. (3.6.25)
Should Lee have just run away? Should he have just refused to fight? Would that have been nobler? Did he have any choice? Are these questions going to stop? This quote hearkens back to the quote at the beginning of the book, where Lee resigned his post in the army, saying that he couldn't fight against his own family and his friends—Virginians, specifically.
"I was trying to warn you. But… you have no Cause. You and I, we have no Cause. We have only the army. But if a soldier fights only for soldiers, he cannot ever win. It is only the soldiers who die." (4.5.61)
Lee admits to Longstreet that there's no hope at this point. He doesn't believe that they can retain their Southern way of life after Pickett's Charge. Just fighting for the sake of fighting won't be enough.