I thought you would say Civil War.

His avatar is of Lord Cutler Beckett, a character from the Pirates of the Caribbean film series, which takes place during pre-American revolutionary times. That fashion was way out of style by the Civil War.
WWII. Because of aircraft, the atom bomb (which led to the hydrogen bomb and modern weapons of mass destruction), proper automatic weaponry, tanks, submarines, medical tech, etc. Civil War era innovations don't really compare to WWII's innovations, really. They improved, but in comparison to WWII's changes it wasn't much.