Shaara switches from person to person, freely entering the minds of different characters at will. Usually, he confines himself to the perspective of one character per chapter, but sometimes he'll make an unexpected leap into someone else's mind. For instance, at the very beginning of the novel, he goes from Harrison's perspective to General Longstreet's all in the same chapter.
This omniscient approach is great for a novel like this, since when you're dealing with a huge, complicated issue like the Civil War, you want to enter into many perspectives as possible so that you can compare them. In this novel, you get to see how a Northern anti-slavery man like Chamberlain sees things, and how a Confederate general like Lee views them, and what an outside observer like the Englishman Arthur Fremantle thinks.
It's sort of like the old story about the group of people who had to enter a dark room with an elephant in it and describe what the elephant looked like based on touch. They each had a different perspective, but when they put their perspectives together, they arrived at a fairly correct picture. That's basically what Shaara's trying to do in this novel.