I ask this question as I’ve heard a few youtubers talk about European reaction to the Civil War. In that History Matters simply said that they were unimpressed and elsewhere that there were lessons to be learned there before WW1.
I’ve also heard that the iron clad ships had a massive impact on Naval warfare. So were these immediately adopted in France and Britain or was it a slower process?
I’m also interested how they observed military tactics, as it would seem strange that a foreign military expert would find it easy to observe what was happening.
In order:
The foreign observers had a generally mixed reaction. The United States Army's artillery branch was generally viewed as being very competent and well supplied and utilized. The infantry on both sides were rated as brave and effective. The level of training of the average soldier did vary between individual armies and that was commented upon. General Bragg was rated highly in terms of having the best drilled army in the CSA by British observers. The use of trenches was not appreciated at the time.
In broad terms, the general foreign view was that the Civil War comprised armies of amateurs that improved over the course of the war. The level of staff and planning was lowly rated by foreign observers. The use of railroads and telegraphs was something that was of great interest.
That said, the level of industrial capacity and mobilization by both sides impressed the observers.
ironclads: The major European powers had already decided to start moving to ironclads prior to the start of the Civil War. However, the speed of transition was greatly accelerated as the US had proved conclusively that it could cheaply, rapidly build, and deploy ironclads that could defeat wooden ships-of-the-line. British naval observers were not just able to see ships after launched, but were able to tour some of the US ironclads during their construction. While they believed that their ships could be competitive against the US ironclads, the fact that they could not state that they could definitely defeat the US ironclads in an engagement was the most disturbing thing. (Essentially, the British Navy was far more concerned with the appearance of invincibility than actually being invincible. They did not want to engage in a war where the outcome was doubtful in terms of affirming naval dominance).
The US Navy's switch to a turret gun system radically changed ship design and gun design with every one of the major powers. The turret design was also pushed by some developers in Britain prior to the Civil War, but the British Admiralty preferred a more traditional gun placement on gundecks, then armored the area around them (called a broadside ironclad). The British government, upon hearing about the Battle of Hampton Roads, immediately approved a test program to test turret designs and stopped ordering broadside ironclads. In the interim, they moved to a central battery design where there were far fewer high powered guns that could pivot over wide ranges, then to a full turret design a few years later. The central battery was better suited for ironclads that used sails for propulsion.
The last question: Foreign military observers were allowed a remarkable amount of access and were treated very well by bother sides in the war. They were often allowed to travel between the two sides as neither side wanted to offend the foreign powers. They were given access to staff and planning meetings and were often given favorable positions to observe the combat. The CSA was very interested in obtaining foreign support during the first couple years of the war, so allowed a great deal of access. The US did not want to offend those foreign powers, so they not only did not complain, but often facilitated the travel of those observers to the south by allowing them to enter through US ports rather than trying to run blockades or travel through Mexico. The US also freely allowed foreign observers in their own operations to show the forces being brought to bear by the US to discourage foreign powers from intervening.
Of critical importance to the US was that it showed the foreign powers that they simply did not have the military capacity to intervene. In this, they succeeded. Britain simply could not win a land war against the US on its own. So, they made it a condition that France would also have to intervene as France would have to handle the land war in North America. France was tied down in Mexico and did not have the manpower or inclination to get into this war. The general consensus of the European powers that was eventually reached was that 1) it would take a firm commitment of all the major European powers - Great Britain, France, Spain, Prussia, Austria, and Russia - to broker a peace and 2) no such commitment was ever going to be reached as Russia was not only supporting the United States, but it would intervene militarily on the US side should Great Britain and France intervene militarily in the US Civil War. It was in Spain's interests for the war to drag out as long as possible as that would keep Cuba safe from US/CSA intervention for a longer period of time.