How It All Goes Down
As the story begins, an eccentric, Shakespeare-quoting Confederate spy named Harrison brings news to Generals Longstreet and Lee: the Union Army is close by. They're totally taken by surprise, since their devil-may-care cavalry officer, Jeb Stuart, has left them in the dark. Lee's troops, particularly a division led by a general named Harry Heth, accidentally get into a fight with Union cavalry near the town of Gettysburg. The leader of that cavalry, John Buford, decides to try to hold the Confederates off and buy time for the rest of the Union Army to arrive.
Meanwhile, over the Maryland border, Colonel Joshua Chamberlain—another Union man—is waking up. He discovers that he needs to convince a bunch of mutineers from another regiment to stay and fight with his, the 20th Maine. Surprisingly, he finds that the old inspire-them-with-a-speech tactic is pretty successful. He lays out the Union Cause—it's all about destroying slavery and aristocracy—to the disgruntled and battle-weary soldiers. They almost all agree to fight. Soon, they're given orders to march into Pennsylvania, headed toward Gettysburg.
On the first day of the battle, Buford manages to hold the Confederates off until crack Union troops led by General John Reynolds arrive. Reynolds is killed shortly after arrival. On the Confederate side, Lee and Longstreet want General Ewell, another commander, to try to capture high ground near Gettysburg from the arriving Union infantry. He totally messes this up, which annoys Lee. Nevertheless, the Confederates have put up a decent fight and killed a lot of Union soldiers. The British observer, Arthur Fremantle, is an enthusiastic cheerleader for their Cause.
The second day is much worse for the Confederates. Lee, his judgment clouded due to poor health and heart trouble, rejects Longstreet's plan to swing around and cut the Union Army off from Washington. The fight needs to be here, he thinks. Instead, he sends Longstreet's forces to attack the Union flank, trying to seize some high ground—Little Round Top—in the process. On the Union side, Chamberlain's troops are just arriving.
Chamberlain's regiment ends up defending the extreme flank of the Union line on Little Round Top in order to stop Longstreet's troops from breaking through at all costs. It's an extremely bloody fight, and Chamberlain loses a lot of men—but he saves the day by launching a bayonet charge after his troops run low on ammo.
Longstreet feels that the Confederates have suffered a near loss, but he's surprised to find that Lee doesn't think so: he thinks the Yankees are close to breaking. On the next day, in fact, Lee wants to launch a major attack right in the center of the Union line, attempting to shatter it. Longstreet senses that this is an extremely bad idea, but can't convince Lee.
On the third day, Chamberlain, still high off the heroic feat from the day before, gets transferred to the center of the line—the quietest part up until that point. Unfortunately, he discovers that his friend Kilrain has died after being wounded the day before. Just after he receives this news, the Confederates start to bombard the Union line, preparatory to their attack. Lee sends General Pickett's division, along with two others, to charge the center of the line: it totally fails, going down in history as the infamous "Pickett's Charge." A Confederate General, Armistead, dies in the battle as he nears the position of his best friend, Win Hancock, a Union leader.
In the end, Longstreet and Lee are broken. They both doubt that they'll be able to win the war, and they feel that the Cause has also died. Chamberlain comes to admire the bravery of the Confederates, but he also becomes steeled in the conviction that the anti-slavery, pro-Union Cause is the right one.
The day after the battle, it's the Fourth of July—which symbolizes the "new birth of freedom" won by the Union side.