What new weapons or improvements on old weapons made the Civil War the deadliest war in U. S, history?

Machine guns and repeating rifles

Explanation:

Machine guns were used in the Civil War. Compared to the powder muskets of the revolutionary war, These guns didn't need to be reloaded as often and as a result could inflict more casualties on the opposing side. They were also an all around improvement, with better accuracy then their primitive counterparts.

s the president shall be commander in chief of the army and navy of the united states, and of the militia of the several states, when called into the actual service of the united states; he may require the opinion, in writing, of the principal officer in each of the executive departments. sooo yeahhh

explanation: