I do strongly doubt that the tactical and operational experiences from 1861-1865 had much value in regard of the problems of 1914-1918.
The machine gun invention, smokeless powder invention (which made the machine gun really useful), widespread introduction of explosive and shrapnel shells, standardization on breech-loaded guns, standardization on quick-firing guns, the rise of technical troops, introduction of automobiles, introduction of spitzer bullets, introduction of open battle order (the version of that period), development of telephones, late industrialization population growth, introduction of railway artillery all came after the USCW.
The development of a general staff, the skilled application of operational and tactical art (such as encircling à la Sedan), fortress networks, central fire cartridges, high population density were pretty much absent and other advances of the time such as standardized arms production were iirc not fully implemented yet as well.
These changes changed the face of modern conventional ground war several times between 1865 and 1914.
The Boer and Russo-Japanese Wars were more highly regarded as lessons before 1914 and that was right. Both were totally inadequate lessons (as were all wars post-'45 in regard to today's state of the art), but still much more relevant than the USCW simply because they were fought with more similarities to the situation of 1900-1913 than the USCW had to offer.
The lessons of 1865 cannot have been too valuable anyway because the U.S. Army quickly turned itself into the apprentice of the French army and learned the "firepower destroys, infantry occupies" attitude.
(The U.S. Navy became apprentice of the Royal Navy, having been late with the Dreadnought and central fire control revolutions).
The USCW (ground) had its interesting facets in regard to telegraphs, railways, marauding and industrial production - but it wasn't interesting in regard to operational art and tactics in general. Napoleon was better - MUCH better.
The use of telegraphs in the Prussian wars +/- 10 years was more professional, while superior operational art offered less time for industrial mobilization (the war was won without throwing huge quantities of newly produced resources at its problems).
Accordingly, I have a rather low opinion of the generals involved and suspect a tunnel vision whenever an American expresses his admiration of a USCW general.
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