https://www.youtube.com/watch?v=mqn0kiYBiMI&list=PL4w-2j6Q0Qj4iJ7GmEoQGWMUBeE5h1GVq
The Brits did a 6 part documentary series on the Spanish Civil War back in the 80’s much similar to ‘The World at War’ and were able to still interview some of it’s aged participants.
It was actually more Spanish than Communist vs. Fascist. Stalin, Hitler and Mussolini were all frustrated with the factions they were backing.
A US Civil War will also be extremely complex, extremely parochial, and will no doubt frustrate any foreign powers who decide to intervene.