It genuinely frustrates me to hear people proclaiming (not arguing, stating it as if it were a given) that tactics in the civil war were outdated due to technological advances. In reality, the technological advances that were gained between the Napoleonic Wars and 1861 were not as profound as many people assume.
There's several factors behind it, and since this is something I've looked into extensively I'll summarize my understanding:
- The adoption of the rifle-musket with adequate training
did change the shape of the battlefield, but it did not do this by making most Napoleonic operational and tactical concepts obsolete. Instead it made it so that firefight ranges could be longer and expanded the tactical menu possessed by infantry.
- During the early Civil War, most long arms issued were
smoothbore muskets, not rifle-muskets.
- As the rifle-musket was adopted, the lack of institutionalized training meant that the expanded range and accuracy of the rifle-musket did not get realized in the Civil War.
This is basically because for a smoothbore musket within about 100-200 yards someone who is well trained and good at aiming is noticeably more accurate than someone who has not been trained, but the difference isn't battle winning. It was an active area of military speculation how much there could be an improvement, and during the 1850s and 1860s only a few countries really realized how much the rifle-musket made a difference (Britain did it best, followed by France, with Prussia coming third).
The difference is basically that it has reached the point where you can't just "point the gun in the right direction" at long range (unlike with a musket, where just pointing the gun
at the enemy will work within the effective range of the musket) - you need to be well trained at range estimation to get the most out of the weapon, and that is something which the two sides of the ACW largely did not do
except in some specialized sharpshooter units (like the 1st and 2nd US Sharpshooters).
- As a result, there is no statistically significant difference between the performance of rifle-armed units and smoothbore-armed units on the battlefield, except for specialized sharpshooter units.
- This means that the infantry part of the battlefield is fundamentally Napoleonic in terms of their limits, so as far as infantry is concerned Napoleonic tactics should still work.
- For artillery, the guns that are available increase the effective ranges a bit (as in, rifles are effective at a somewhat longer range than 12 pounders from Napoleon's day, 12 pounders replace the 6- and 8- and 9-pounders of Napoleon's day) but the range increase is comparatively small because, well, cannister is the same as ever and a cannonball becomes less dangerous the further it's travelled, while US rifled artillery does not have good percussion fuzes and is not sufficiently accurate to change the shape of the battlefield. It's an incremental improvement.
- For cavalry, the basic problem with cavalry employment in the Civil War is that there wasn't good battle-trained cavalry able to
attempt shock action on anything like a useful scale (as in, regimental or brigade saber charges) until the second half of the war, and certainly there's almost never the same cavalry proportions that Napoleon had available (and when it is it's sent off on a raid). There are several cases in the late war where saber charges happen.
What this means, overall, is that the battlefield environment and the constraints on it have not actually invalidated the Napoleonic battlefield. A lot of the time when Napoleonic tactics or operational techniques are used (and they're not always attempted) it actually works pretty well, and when it doesn't this is usually because of a failing in the troops rather than the tactics being outdated.