He had never believed in this invasion. Lee and Davis together had overruled him. He did not believe in offensive warfare when the enemy outnumbered you and outgunned you and would come looking for you anyway if you waited somewhere on your own ground. (1.1.53)
Longstreet thinks the South should be playing tenacious D… by which we mean tenacious defense, of course. He thinks the Southern troops aren't powerful enough to attack the North on its own turf. Lee would probably argue that it's better to try to force a conclusion by beating the Union in its own territory and winning foreign support for the South in the process, instead of letting the North slowly grind down the Southern defenses.
"We are good men and we had our own good flag and these goddamned idiots use us like we was cows or dogs or even worse. We ain't gonna win this war. We can't win no how because of these lame-brained bastards from West Point, these goddamned gentlemen, these officers." (1.2.81)
Bucklin, one of the Mainers refusing to fight, complains to Chamberlain about the way the officers have been treating him and the other men. The Union soldiers liked McClellan, the general responsible for training the army and making it the fighting force it became, but they don't care much for many of the commanders who replaced McClellan. The problem with McClellan, though, in Lincoln's eyes, was that he was reluctant to attack the Confederates.
Nothing quite so much like God on earth as a general on a battlefield. (1.2.108)
Joshua Chamberlain, being a colonel, sees the highest officers, the generals, as godlike. They hold the fates of human beings and of nations in their hands. But a general might feel that he himself is a pawn of higher powers—as General Lee does.
The war had come as a nightmare in which you chose your nightmare side. Once chosen, you put your head down and went on to win. (1.4.151)
According to Shaara, Longstreet wasn't that big on the Southern "Cause," even though he was fighting for it. Here, he sees the war as pointless slaughter—but, now, since he's picked his side, his duty is to follow through to the bitter end.
Napoleon once said, "The logical end to defensive warfare is surrender." (2.1.68)
Lee is a general akin to Napoleon, whereas Longstreet favors defensive war. In fact, as Shaara points out in the book's afterword, Longstreet was ahead of his time, anticipating the prolonged trench warfare used during World War I.
"We must attack," General Lee said forcefully. "We must attack. I would rather not have done it upon this ground, but every moment we delay the enemy uses to reinforce himself. We cannot support ourselves in this country. We cannot let him work around behind us and cut us off from home." (3.3.21)
Lee is trying to justify his decision to attack the Northern line instead of following Longstreet's plan. (Longstreet wanted to swing around the Union Army and cut it off from Washington.) Since Lee's plan ended with the disaster of Pickett's Charge, maybe Longstreet was right, after all.
"To be a good soldier you must love the army. But to be a good officer you must be willing to order the death of the thing you love. That is… a very hard thing to do. No other profession requires it. That is one reason why there are so very few good officers. Although there are many good men." (3.3.114)
This is Lee talking. Although Lee is known for being a brilliant general, he was actually very aggressive; he wasn't afraid to send soldiers off to die. At the same time, he was apparently very concerned for his soldiers. For example, after Pickett's Charge (this is in the book, and it's also historically accurate), Lee rides beside his retreating soldiers telling them that it was all his fault.
He rode purposefully, slowly off into the dark feeling the swelling inside his chest like an unexploded bomb and in the back of his mind a vision of that gray rocky hill all spiked with guns, massed with blue troops at the top, and he knew as certainly as he had ever known anything as a soldier that the hill could not be taken, not any more, and a cold, metal, emotionless voice told him that coldly, calmly, speaking into his ear as if he had a companion with him utterly untouched by the rage, the war, a machine inside wholly unhurt, a metal mind that did not feel at all. (3.5.26)
Longstreet isn't susceptible to delusions—he knows when he's lost. Part of his brain is able to maintain distance and keep things in perspective. After losing Little Round Top to the Union (thanks partly to Chamberlain's men), Longstreet is convinced that the Confederates never get it back. Although Pickett's Charge hasn't happened yet, Longstreet recognizes that the battle might be over.
"God in Heaven," Longstreet said, and repeated it, "there's no strategy to this bloody war. What it is is old Napoleon and a hell of a lot of chivalry." (3.5.151)
Since Longstreet isn't into romanticizing the Southern Cause—he's no Gone with the Wind character—he thinks Lee's Napoleon-style tactics are flawed. He would rather fight a more defense-oriented war, instead of parading into the North and attempting to defeat the Union with one momentous battle. It's as if the South is living in the past, fighting a war in a style almost a hundred years old.
Chamberlain closed his eyes, slept again. Opened them and lost all sense of time, had been sleeping since Noah in the sound of the guns, had slept through the mud and the ooze, and the thousands of days since Creation, the guns going on forever like the endless rains of dawn. The earth was actually shuddering. It was as if you were a baby and your mother was shuddering with cold. (4.3.54)
As he survives that huge artillery bombardment before Pickett's Charge, Chamberlain imagines the war as a calamity convulsing the earth, which is like a pregnant mother. It seems to last forever. But Chamberlain has already gone through his own climactic fight at Little Round Top—he's mainly an observer during this part of the battle. The imagery of baby and mother shuddering in the cold strengthens the feeling of violence in this scene and reminds us of some of the human costs of war.