The cause of the civil war

I started reading a book containing a set of essays on the American Civil War.   The first one is on why the war happened.   It claims that the reason was (as the popular idea has it) slavery, but makes a comment that I was looking for -- why the North decided to oppose slavery.

I can understand why and that the South was sufficiently attached to slavery to succeed in response to a threat.   What is less clear to me was why the North needed to oppose it and why it would fight to stop it.   I understand the idea that the people of the North were so moral that the idea that slavery would continue in this country was abhorrent -- it's a nice idea, but I doubt most people in the North would die for this ideal.   Certainly some would (e.g. John Brown), but the ordinary person -- I have a harder time imagining this.

However, the fight was not only about the morality of slavery, but also about the difference between "free labor" and slavery.   This is a big deal.   The idea was (probably correctly) that slavery and "free labor" were incompatible.   I think what they mean by free labor is what I would call working for wages (more learning to come).

This was the time of the factories, in which people were working 12+ hour days from early ages to produce goods for (miserable) wages.   I read once that these people were actually less expensive for the factory owners than slaves would have been.   They worked for very little money, but the owners had no capital tied up in hiring (versus buying) them, plus the owners did not need to provide for their room and board.    There was a large supply at the time, since Europe had a sizable excess population, many of which decided it was better to come up with the price for the trip across the ocean for the chance at a better life in the new world.

This "free" labor -- free in the sense of not being slaves -- would move into an area and take jobs if there were jobs to be had.   If the possible jobs were held by slaves, they would not move in.   If they had no jobs to go to, this immigrant population would be a liability, rather than assets, to the new country.    Thus, the management class of the North, who were making fortunes from this immigrant labor, had a motive to want to encourage this to continue and grow.

This was also the time when Britain decided to stop the African slave trade.   It was indeed morally right, but the same kind of analysis applies.  Britain had a large population of people it had no ability to employ.   Stopping the slave trade would have the effect of providing jobs --- in the colonies -- for this lower class.

I am totally on board with the idea that slavery is bad, but less so with the idea that the average person would feel as passionately about it as John Brown did.   One's economic opportunities, otoh, are much more existential.