Hi, I wrote a sorta lengthy post for the question "How and why did people accept nationalism when it first emerged?" and I think it should answer your question.

Ultimately though, as I hint at in the thread, every nation has its own specific relationship with what we can call "nationalism"; by that I mean the rise of a universal national consciousness. The story of Germany is the one I tell because it's probably the most relevant and easily studied (imo) much in the same way every genocide course goes over the holocaust because of how relatively straightforward it is. Once we start going into French (though this is, in my opinion, the second easiest one to grasp) or Russian or Slavic or English or expanding outward to, say, Japanese or Palestinian or Kurdish identities we get into a giant shitstorm of "what is going on".

I know many hate the cop out of "it's too complicated for one answer" but in this case it really does require further specification of what national group you're speaking about if you want any sort of depth. Nonetheless I hope the above thread does a good job of showing the explosion of nationalist thought for the first time in France & Germany and gives you a decent idea of why it grew so rapidly in those regions!

Was it Social Darwinism?

Though I am curious why you're bringing up social darwinism in this discussion; it seems quite out of place.