I just finished Paddy Griffith's book on battle tactics of the American Civil War, Rally Once Again. He comes to some interesting conclusions that don't match up with the ideas prevalent in other books I've read on the conflict.
I won't do justice to his argument, but I will try to sum it up. Griffith argues that the ACW was not the first war using modern tactics (trench warfare, no more cavalry, etc) but the last using Napoleonic tactics.
Griffith argues that the improved firepower of the Civil War did not make it a modern one. He says that cavalry could still have been very relevant, and that it was, even with the sabre, when properly used, that artillery still could operate aggressively and forward, and that even with rifle muskets infantry still slugged it out at Napoleonic ranges.
He also argues that the common fortifications of the Civil War were not brought about by necessity and increased firepower, but by a engineer mentality that set in as the armies tired of battle in late 1863 and 1864.
Griffith believes that shock tactics were poorly understood by most officers, and therefore most battles were rather indecisive bloodbaths, bloody by nature of their length and not because of increased firepower. Decisive victories could have been possible, he argues, had tactics like the "Zouave Rush" been implemented on a large scale.
He also argues that technological innovations had less affect on the armies than is usually believed. Railways and steam power offered mobility, but Griffith says these gifts were usually made moot by the cautious nature of generals. It is to 1870 we must look for the first modern war, says Griffith.
Next to these conclusions in my book, a previous reader has written "Crud", "Bunk" etc. I was surprised by Griffith's conclusions, because they're the opposite of what I have usually learned, as I said.
So, tl;dr, do Griffith's claims have any truth to them, or is he way off the mark?
Griffith's claims have some truth to them, but you shouldn't accept them uncritically either. He's well respected in his field, and taught at Sandhurst, and is in a small way a pioneer in the study of battle tactics. However his work on the American Civil War does overturn conventional wisdom. Plus he ruffles feathers being a Brit, but perhaps the outsider's perspective is useful.
I haven't myself read Griffith. It's on my list but I wanted to read a few more even-handed accounts of civil war tactics first.
You might try a more descriptive, less controversial work to help form your opinion of Griffith. I liked Brent Nosworthy's "Bloody Cruicible of Courage: Fighting Methods and Combat Experience of the Civil war."
No the American Civil War was not the last war to use "Napoleonic Tactics" so to speak. The Second Schleswig War (Fought during the Civil War) the Austro-Prussian War (fought a year after the Civil War) the Paraguayan War (fought from 1864-1870) the Japanese Civil War (fought from 1863-1869) the Franco-Prussian War (1870-1871) were all fought this way, but the true death of these tactics was the opening months of the Great War. The Invasion of Belgium and France and the Battle of the Marne had soldiers dressed like they were straight out of Napoleon's Army and many primary accounts state that infantry regiments would line up and march shoulder to shoulder right into machine guns. Cavalry charges were still the norm and France still even used Cuirassiers (Cavalry Squadrons dressed in metal armor). With the wide spread use of trench warfare beginning in the closing months of 1914 'Napoleonic Tactics' truly died.
Furthermore he's wrong to assume the generals were just cautious so the tactics weren't able to be used correctly, there are many examples of Napoleonic Tactics and charges demolishing armies fighting capabilities with very little casualties on the defending side. Take Pickett's Charge for example, that wasn't very cautious and instead of resulting in a victory, like Griffith says it would have, it was such a defeat that the Confederate Army never recovered, losing half of their 12,500 men in the charge.