Well you are wandering much too far afield in answer(if that is what you are doing) to my small point.If more modern technology make it harder to win wars quickly, then why did the Prussians Beat the Austrians in a few weeks in 1866?
And French also pretty quickly in 1870?
I will argue that the size of the armies matter...
If there are more than one field army, destroying one do not end the war. And that is one of the big issues in the west in 1914.
And why the armies are fighting.
In some wars you can beat the one enemy field army and rout it... and that is basically the end of the war.
This is common in wars that are limited where the political issue is a matter that only really matter to the political leadership and maybe the middle class.
1866 is a good example of this. The Austrians Lost the main battle and then they accepted that they where no longer part of Germany... and that was it.
But in wars that are (close to) total wars, then you pretty much need to take control of the entire enemy country with garrisons across the country... and they will still fight you.
Spain during the later part of the Napoleonic wars are an example of this. The French was fighting much of the Spanish population.
And I would say that from 1812 in Russia and to the end its the same in north and central Europe.
By 1813 the war had become a peoples war between the French vs. the Germans and Russians.
Defeating a field army in no longer something that win wars for Napoleon.
The franco-prussian war show both types. The Prussians pretty much destroy all French field armies and expected the French to ask for terms.
But they do not... and it change from a limited war between two states* over an perceived insult to a war between two nations that drag on for much longer than expected. And basically result in a French civil war.
And cavalry. What was the issue in 1861-65 was the lack of proper trained and equipped cavalry...
It took 2-3 years to train a regiment of cavalry. That is why we don't see federal cavalry do proper saber charges until late in the war. And when they start to do so, they are usually very effective.
And the other issue is the terrain. The typical civil war battlefield was simply less open than a typical European battlefield. So even if the cavalry had been there, it would have been harder at most battles to use it like Napoleon or Blücher would.
The rifle musket is irrelevant. Throwing cavalry at infantry in good order was a bad idea in 1805. (and in 1400 for that matter)
and it still was in 1861.
And throwing them at disordered infantry or even better infantry retreating was a good idea in both...
I am only saying that I believe Lee was psychologically either knowingly or not, to the Napoleonic example of a single campaign or battle deciding the outcome of a war, wherein the deciding factor might be the brilliance, or luck, of a single commander. To the extent Lee accepted such a concept, I say that the increasing complexity of modern weaponry and its production, made such a concept/belief obsolete in the ACW.
P.S. as factoid, I will note that most studies of wars for the Unification of Germany by Prussia in the 19th Century, is usually ascribed to the Prussian Army being better armed and trained than their varied opponents and as a result, IMO, there could be no single stroke of 'genius' but a continuous grinding down of the opposition by steady application of Blood and Iron.