Ok maybe "northern aggression" is a bit too controversial word choice as this history is debated and some people are going to chime up and express their own personal expertise and disagreements. But this was my reading of it and also how it was taught to me uni, but it doesn't matter, we can agree for purposes of the main point that slavery (a moral cause) was a pretext and even today, it's hard to find people who care about what is right. I know this, because I'm dealing with blatant abuse.