I have recently seen the Danish made series (1864) which is about two brothers fighting in the Second Schleswig War between Prussia and Denmark. It's a really gory and "raw" series about how brutal war was back then, but a very short scene in this series which really made me think a lot is that you at one point see the Prussian army match and some of the soldiers are speaking and one of them is really against this war and he felt it was unfair that the leaders of each country abused the people and stole their wealth and also used them as tool for wars only they wanted, you know all that kind of stuff, another soldiers questions it and asks if he is afraid of having this views of these things, but he also asks for his name which he replies "Karl Marx" and I have been digging a little, but I'm not the best at finding info like this, but I can't find any info about Karl Marx actually fighting in this war, to me it would make sense if he did and this is why he got his view to write the communist manifesto and then creating communism, which had a big impact on modern history.
I just felt it was a interesting that the ideology there had a big impact of the second world war and it almost started ww3 was created in a conflict which was started by little Denmark which in modern history has really always been a push over.