Grant and Sherman were the first generals to grasp that Napoleonic tactics were outdated and their was a new form of warfare emerging, Total War. European observers completely missed it because they dismissed it as American amateurism, but the war in late 1864-early 1865 was a complete preview of what the Western Front would look like in WW1.
Lee realized even before First Manassas that the South could not win a war of attrition. That is why he continually sought to go North. Stonewall Jackson's writings show a man who knew that the Napoleonic warfare was no longer viable. His style of fighting changed warfare as well.