It was that there was unprecedented bloodshed because they used a new, deadlier type of bullet with more accurate weapons. This was particularly deadly because the tactics they were using hadn't adapted to the new weapons and soldiers did a lot of standing and shooting at one another.
This stood out to me because a decent part of our success against Britain during the revolution was because of hit and run tactics. It also stated that the south was outnumber more than three to one. How did the south deal with being so outnumbered if soldiers were standing and shooting? It seems like those tactics would rely on having more soldiers firing more bullets.