According to CNN, "the line of succession falls to the speaker of the House, then to the Senate president pro tempore, then to Cabinet members."
Source: http://www.cnn.com/2013/08/27/us/succession-presidential-and-vice-presidential-fast-facts/
The American Civil War became known as the first modern was because it was the first war to ever use modern weapons. While other wars such as the American Revolutionary War used gunpowder, the American Civil War used weapons such as the Gatlin Gun, used by General Custer, repeater rifles and pistols, and grenades.
