The important thing to understand is how atypical trench warfare was in WWI.

The Western Front, Gallipoli (April 1915 - January 1916), Salonika (October 1915 - September 1918) and the Italian Front (May 1915 - October 1917) were the ONLY fronts to experience Trench Warfare. Moreover, the latter three fronts I mention require qualification: Gallipoli lasted almost a year, and the stalemate here was imposed by terrain and lack of resources; Salonika had much the same problem, but even then, the perimeter shifted a fair amount between 1915 and 1918; and I don't think I would flatter the Italian experience with the term 'Trench Warfare', as this was mountain/quasi-trench warfare, again largely imposed by terrain, and above all over come by the Germans in 1917, leading to fairly mobile fighting till the end of the war.

The Western Front, despite some insistence, was NOT condemned to Trench stalemate because the 'technology of DEFENCE' outstripped the 'tactics of OFFENSE'. This isn't Dungeons and Dragons, you don't role a die for attack or defence. Machine Guns, Artillery, Rifles, ALL of these were used by defenders AND attackers. Attackers attacked, more often than not gaining a foothold, before the defenders COUNTER-attacked.

It had always been acknowledged that the defender had an advantage, and the 'golden ratio' was that the Attacker should have at least 3 men for every 1 of the defender's. Trenches were also NOT new, AT ALL, to European Armies. Trenches were widely used in siege warfare, since the days of Homer, and there were examples such as the Lines of Wissembourg and the Lines of Torres Vedras in the 18th and 19th centuries respectively. The mid to late 19th century saw trenches and other such defensive field works used in the Crimean War, the American Civil War as you mention, the Russo-Turkish War (1877-1878), Russo-Japanese War, and the Second Anglo-Boer War. At Gravelotte and St. Privat in the Franco-Prussian War French troops behind cover had inflicted casualties as high as 60% on Prussian companies - and yet Prussia won the war.

Trench Warfare had everything to do with the size and armament of the armies, IN RELATION TO THE AREA IN WHICH THEY WERE FIGHTING. The Eastern Front had large armies and ALL the technologies of the Western Front, but NEVER succumbed to the trench stalemate of the West; in fact, fighting in the East, AND in the Middle East, was quite mobile. This had to do with the immense distances in the east, which precluded any large scale field works on the scale of the western front. The difference in the West? Large armies, about 1 million Frenchmen facing a roughly equal amount of Germans, supported by c. 200 000 British and Belgian troops by the end of 1914, facing each other on a frontline squeezed between the channel and the Swiss frontier. Armies of that size with formidable firepower could easily hold extensive sectors of front, while the need to mass forces to attack meant that operational surprise was virtually impossible.

Three technological hiatuses made the stalemate, when it did develop, difficult to break:

1. Communications Technology: Generals were still dependent on runners, drums, bugles, trumpets, semaphore, heliographs and various 19th century examples of Battlefield Communications, to fight a 20th century war; lacking battlefield radios, not widespread until the 1940s, their ability to control their forces in battle and make snap decisions was greatly hampered.

2. The Internal Combustion Engine: transport based on the internal combustion engine was already beginning to displace horses, but for a mobile arm of decision, mobile firepower, and troop transport on the battlefield, the horse still dominated. Tanks would not appear until 1916, and left much to be desired.

3. Aircraft: Aircraft were very useful as artillery spotters, but were unable to greatly affect ground operations with ground attack and bombing missions. Unlike in WWII, armies on the defence AND offense could reinforce themselves with operational reserves, without the interference of airstrikes, further aiding the stalemate.

Breaking the stalemate, which had already begun by 1917, would require sophisticated infantry tactics, combined with a PRODIGIOUS amount of firepower from artillery. The former developed much quicker than most people assume; the latter really was not present for both sides of the Western Front until 1916.

Trenches were not alien to Europeans; the issue was that Trenches on the Western front were like nothing before in history: a formidable position, from EITHER side, that HAD NO FLANK TO TURN. Amphibious landings were risky, though they were planned like Operation Hush, and attempted like Gallipoli; Switzerland was neutral, and smack dab in the Alps. There was NO WAY to confront the Trenches save to go THROUGH them; no easy task.

So what could the US Civil War teach the European armies that they didn't already know? Not much. While the centenaries of the Napoleonic wars were reaching their end in 1914, so were the FIFTIETH anniversaries of the US Civil War; it was WELL in the past for modern observers. The Franco-Prussian War had demonstrated that he who mobilized the LARGEST army the FASTEST and with the MOST FIREPOWER, would triumph; as the confederate cavalry general Nathan Bedford Forrest was alleged to have said, victory would go to he who was there "the fustest with the mostest". The siege lines around Petersburg and Richmond, and various other examples of confederate defensive actions, were overcome at great cost, but they WERE overcome. Europeans expected high casualties and fierce fighting, but in the words of British Military Historian Sir Michael Howard, future wars were expected to be "BIG, BLOODY, and DECISIVE." The Germans expected the war to last at most 2 years, hence why 1916 was such a pants s***ing moment for them!

Does this sort of answer your question?