Complex sentence: In later stages of the battle, many Union casualties were black soldiers killed by Confederate bayonets and musket fire even after surrendering--a direct violation of the rules of war prevalent at the time (the Lieber Code).
Split into simpler sentences: In later stages of the battle, many Union casualties were black soldiers killed by Confederate bayonets and musket fire even after surrendering. All of this was a violation of the rules of war prevalent at the time (the Lieber Code).