Symbolism, Imagery, Allegory
The Killer Angels ends when a storm strikes after the battle, raining down on the corpses:
The light rain went on falling on the hills above Gettysburg, but it was only the overture to the great storm to come. Out of the black night it came at last, cold and wild and flooded with lightning. The true rain came in a monster wind, and the storm broke in blackness over the hills and the bloody valley; the sky opened along the ridge and the vast water thundered down, drowning the fires, flooding the red creeks, washing the rocks and the grass and the white bones of the dead, cleansing the earth and soaking it thick and rich with water and wet again with clean cold rainwater, driving the blood deep into the earth, to grow again with the roots toward Heaven. (4.6.27)
The next day, it's the Fourth of July. Shaara subtly uses the rain and imagery of fertility to imply that the blood shed at Gettysburg will cause—to quote Lincoln's Gettysburg Address—a "new birth of freedom" in America, ending slavery and fulfilling the pledge of liberty made in the Declaration of Independence. As Thomas Jefferson said, "The tree of liberty must be refreshed from time to time with the blood of patriots and tyrants."
The Fourth of July symbolizes the promise of freedom that the Union soldiers are trying to realize. The difference is that after Gettysburg, that promise is now closer to becoming a reality for all Americans. African Americans are much closer to escaping slavery, and the Southern slave-owning aristocracy is in its eclipse.