For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
The armies try to get into position. A spy, Harrison, lets the Confederates know that the Union is getting closer. Lee and Longstreet scramble to make gains, but General Ewell wimps out and refuses to try to take two of the most important hills on the battlefield. It's not all fun and games for the North, either: one of their highest ranking generals, John Reynolds, dies on the battlefield.
It's becoming clear that Lee needs to get his groove back: he's been dealing with sickness and serious heart trouble, and he's just not feeling too hot about anything. He plans to use Longstreet's soldiers to take high ground on the flank of the Union position at Little Round Top.
On the Union side, Colonel Joshua Chamberlain and his men get sent to defend Little Round Top from the Confederates. As he maneuvers into position, Chamberlain and his soldiers find and help a runaway slave. Chamberlain muses on the causes of the war, which he discusses with his friend Kilrain. Basically, they think it's all about slavery, and they wish the South would just admit to that.
Act II
The fighting at Little Round Top is incredibly intense. The Confederates try to take the hill, but Chamberlain's troops, who are defending the very end of the Union Army, battle aggressively—they even charge with their bayonets when they run out of bullets.
At the end of the day, the Union keeps the hill, though Chamberlain's friend Kilrain gets fatally wounded in the process. Longstreet is doubtful about Lee's approach: he wanted to swing around and cut the Union Army off from Washington, D.C., but Lee wanted to launch an attack. Longstreet grows even more skeptical as Lee orders a major offensive against the center of the Union line—what will later be known as "Pickett's Charge."
Act III
Pickett's charge turns out to be a major disaster for the Confederacy. Pickett's troops are slaughtered, and the Confederates' huge artillery bombardment doesn't work as well as expected. Lee confesses that this was all his fault and admits that the Confederates have lost their cause. Chamberlain gains a new perspective on the war: he respects the bravery of the Confederates while realizing that the true cause of the war is the abolition of slavery.