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 rifles, even if muzzle-loading, could do.
Militaries are eternally building themselves to fight the last war. When they do learn from their experience, quite often the lessons learned are the wrong ones.
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.
I'd say the weapons completely out matched anyone's ability to cope with them until quite late in the war. By that light, I'd say nobody alive had experience fighting a modern army.
The US has always had a charming dread of standing armies.
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 rifles, even if muzzle-loading, could do.
Militaries are eternally building themselves to fight the last war. When they do learn from their experience, quite often the lessons learned are the wrong ones.
[1] https://en.wikipedia.org/wiki/Battle_of_Sedan
[2] https://en.wikipedia.org/wiki/Siege_of_Petersburg