Head-on “bayonet charges” of Napoleonic character remained a preferred tool of Civil War generals almost (if not absolutely) to the end. Not that such charges always worked through that way. A major difference between the technologies of the Napoleonic period and that of the Civil War was the development of effective mass-issue muzzle-loading rifles in the 1840s/'50s. The critical difference between smoothbore muskets and “rifle-muskets” was not a matter of rate of fire or of accuracy (although a truly skilled marksman could achieve accuracy with a muzzle-loading rifle over a considerable distance). That critical difference was in the matter of range. Smoothbore muskets in volley fire were generally effective over ranges of (roughly) 75 - 125 yards; rifle muskets could achieve the same over 400 - 500 yards, and could kill over considerably longer distances. This change drastically reduced the effectiveness of the traditional bayonet charge. The longer range allowed trained infantry to loose off four or five shots on enemy troops within effective range where smoothbores could only have achieved one shot. The effect of this was that instead of having to bear one volley of musket fire from about the limit of their charging range, they might have to endure four or five volleys from a distance stretching well outside their charging range. Equally, if the defending infantry decided it expedient to run away in the face of superior attacking forces, they would have to decide this earlier with the alternative of being shot in the back.
Ordinary soldiers and junior officers seem to have adapted to this situation with more flexible tactics where possible. Not surprising - it was they in the old fashioned, head-on charge, were likely to be killed. However, such flexibility was not always possible - as, for example, where a large head-on charge was ordered on an uphill, heavily-defended enemy position. Arguably, such attacks should not have been ordered in the world of rifle-muskets. But there are instances of such advances at both Chancellorsville and Gettysburg, Pickett’s Charge being the most famous and disastrous. Curiously enough, the battle for the Round Tops was a reasonable example of more flexible tactics employed on both sides, with the Union troops under Chamberlain, in particular, employing a form of “area defence” that would have been familiar to some commanders in the later part of WW1.
All in all, the comparison of the Somme with at least some Civil War battles in tactical terms is not at all without validity. Best regards, JR.