This is specifically in reference to the American Civil War, but after a second thought I realized it would be interesting to see if other examples helped shape how countries fought in it.
For specifics, I know that within the American Civil War that trench warfare was used at times and (at least as an American) we see it as a big precursor to the Trench Warfare seen in WW1. I wanted to know if countries that experienced this warfare earlier on had any sort of advantages within the war or battles fought.
Within my thought process I'd think that countries such as the United States would have a bit of an advantage even while joining late war since they'd have at least some basis on what to do (excluding information we'd know about tactics and strategy from allies and reports), but I'd like to know if this thought process is true and more specific information whether I be right or wrong.
As /r/AHourseNamedPhil has asserted, the American Civil War drew little attention from European militaries and offered even fewer lessons for the future of war. All that said, the conflicts between the American Civil War and the First World War that involved European powers did offer some glimpses of what was to come and European militaries did draw from those conflicts. For the French, the Franco Prussian War 1870-71 suggested that future wars would be wars of mobility. During that conflict in which France suffered a humiliating defeat, Prussian armies outmaneuvered French ones using railroads, giving them tactical advantages. As a result, military reforms after that conflict saw the French largely do away with heavy artillery pieces in favour of smaller, lighter and faster firing guns. This led to the development of the French 75mm field gun. Rapid firing and light weight, it was designed to be able to be moved rapidly and provide rapid fire support for advancing infantry. While an excellent weapon, it proved to be wholly inadequate when the First World War broke out and trench warfare developed. Because it fired a relatively lightweight shell on quite a flat trajectory, it was unsuitable for use against earthworks. It was also out ranged by German guns and because the French had done away with most of their heavy guns, they didn't have anything until 1916 to provide long range fire support. The interim solution was to strip the French forts of heavy artillery which later left them vulnerable when the Germans assaulted them during the Verdun Offensive.
The British were somewhat better off. The Boer War 1899-1901 had shown that British artillery was being outclassed by the faster firing artillery provided to the Boers. Then in 1905, British observers to the Russo-Japanese War saw the effects of artillery on earthworks and recognised that a greater weight of shell, fired at a higher trajectory was required. This resulted in the development of the QF 18-Pounder which served the Royal Artillery and dominion armies throughout the First World War. Although initially deployed in the open to provide direct fire, counter battery fire from German heavy guns forced the field guns to move further back and behind slopes to hide them. From thereon they provided indirect fire, something the Royal Artillery was not particularly experienced at and so had to draw on the experience of the Royal Garrison Artillery to provide. The Royal Artillery also benefited from the development of the 4.5-inch Howitzer which served throughout the war providing plunging fire which was excellent for lobbing shells into trenches and behind earthworks.
Source:
Firepower by Shelford Bidwell and Dominick Graham
The American Civil War was mostly ignored outside the United States, at least as far as studies of it's campaigns went. European military thinkers at the time largely dismissed it as a war waged by unprofessional mobs. While that criticism was a bit too harsh perhaps and more true of the first half war of the war than the second, it also wasn't entirely without merit.
The U.S. in many respects was still a backwater at the time, it started the war with a small army meaning that both sides only had had a tiny number of regulars to call upon, needing to fight the war with volunteers enlisted for the war -whose training & equipment was quite often not up to the standards of the armies fielded by the great powers in Europe - and the shortage of available officers also meaning that sometimes command of divisions, corps, and even armies were being given to political officers with dubious military experience or hastily promoted junior officers who also didn't have the experience. While both produced their share of stars, plenty also ran face first into the Peter Principle. There were also some organizational flaws for both armies.
There is no teacher quite like experience however and while the first half of the war was often amateur hour, by 1864 both armies were very battle-hardened and the wheat had been mostly seperated from the chaff among the officers, and as a result both were now campaigning with a proficiency that would have been up to a European standard. It's here particularly where there may have been some lessons to be learned outside the U.S., as the fighting around Petersburg was in some respects an early harbinger of WW1, and Sherman's March to the Sea was very modern in some respects, particularly in contrast to some of the more "Napoleonic" campaigning earlier in the war. But minds had already been made up, and by that point few were paying attention.
It's not just the American Civil War by the way. Arguably an even bigger harbinger of what was to come was in the Russo-Japanese War, but tht was also dismissed as being of little to value for Western European military theorists, because of racist attitudes towards the Japanese and Russia to some extent also being viewed as a less civilized "other."