Earlier quoted context omitted.
Europe thought that WW1 would be a repeat of Sedan[1], not Petersburg[2]. The generals of the Civil War studied Clausewitz and Napoleon, and the troops went to war with smooth-bore bronze cannon and Napoleonic fire drill. They were largely tempered in the brief, not-too-bloody, and glorious Mexican-American War. It took them a few years and tens of thousands of casualties to learn what havoc accurate, long-range rifl…
There exists other theories. http://johnsmilitaryhistory.com/cwarmy.html The main reason why the civil war was bloody was because it was fought by armies with no great military history or traditions. There were no hussars, no cuirasses. Neither army had experience fighting a modern enemy. Of course it became a bloodbath, like watching two drunks punch each other tired, no skill.
The US has always had a charming dread of standing armies.