Chamberlain said nothing. He was thinking: How do you force a man to fight—for freedom? The idiocy of it jarred him. Think on it later. Must do something now. (1.2.41)
So you draft people and force them to fight for the freedom of other people—it's kind of a paradox, right? Well, there was a lot of resentment about this in the North, because if wealthy people were drafted, they could pay "substitutes" to fight in their place instead. Poor people didn't have that option. In that sense, some of the inequality that characterized the South also characterized the North—but how else, Chamberlain asks, can the problem be fixed than by going to war over it?
Truth is too personal. Don't know if I can express it. He paused in the heat. Strange thing. You would die for it without further question, but you had a hard time talking about it. (1.2.118)
Chamberlain understands a deep truth about the United States and about the evil of slavery, but he finds that it's harder to convert this understanding into words than it is to feel it and know it at an intuitive level. Are most of our principles like this, in a way? What happens when we try to put them into words? Do they lose something?
"This is a different kind of army. If you look at history you'll see men fight for pay, or women, or some other kind of loot. They fight for land, or because a king makes them, or just because they like killing. But we're here for something new. I don't… this hasn't happened much in the history of the world. We're an army going out to set other men free." (1. 2. 140)
Joshua Chamberlain states the Union case for the war, and it convinces almost all the formerly mutinous members of his regiment to pick up their guns and fight again. The Confederates would totally disagree with what Chamberlain is saying—they would claim that the Union was trying to dominate the South economically—but actually, the South was trying to expand slavery into new territories, like Kansas and Nebraska. Lincoln strongly opposed expanding slavery to new places, and almost all serious historians agree that this is why the South rebelled.
Longstreet remembered a speech: In a land where all slaves are servants, all servants are slaves, and thus end democracy. A good line. But it didn't pay to think on it. (3.5.199)
Longstreet thinks it doesn't pay to think in this way, because doing so could undermine the Confederate cause. The Confederates are fighting to secede from a democracy in order to retain slavery—under the guise of preserving states' rights. But will the Confederacy remain a democracy if it's founded on the wishes of aristocratic plantation owners who hold slaves? Since the answer is probably "no," Longstreet decides he just doesn't want to think about it.
"Win, so help me, if I ever lift a hand against you, may God strike me dead." (3.5.220)
Many Southerners joined the Confederate side because they didn't want to fight their friends and family. Unfortunately, General Lewis Armistead—who speaks this line—actually ends up fighting against his best friend, General Winfield Scott Hancock, who fought for the Union. In this quote, Armistead makes a vow to Win that he will never attack him—which he breaks by charging directly into Hancock's line during Pickett's Charge… and, well, we don't want to rub it in, but he is struck dead.
"Will you tell General Hancock, please, that General Armistead sends his regrets. Will you tell him… how very sorry I am…" (4.4.112)
Armistead has broken his vow to Hancock and has violated some serious principles about not fighting your close friends. As he dies, Armistead sends his regrets to his friend for breaking his vow to him.