Northern businessmen never spoke with one voice. Pennsylvania iron and steel men really wanted tariffs to protect themselves from foreign competition.
New York businessmen were often involved in shipping, and those who were had no great love for tariffs. They also had no love for war with the cotton growers who gave them so much business.
New England mill owners were conflicted. Protective tariffs may have sounded like a good idea to some of them, but they got their cotton from the South and didn’t want to antagonize Southern interests.
Political views cut across these categories, though. Some capitalists and industrialists were ardent abolitionists. Others had no use at all for abolition, and only wanted to keep the country together on almost any terms.
I really doubt any of these groups was the main driver of western expansion. Some capitalists, industrialists, and railroad men benefited from settling the frontier, but the main impulse for expansion was always the land hunger of agricultural interests — very much including Southern planters.
I will agree with you that NYC especially was much more willing to strike deals with the South and was much less interested in antagonizing Southerners. The two were business partners. NYC knew full well which side its bread was buttered on.
New England Mill owners generally supported high tariffs. They found it extremely difficult to compete against Lancashire. The British Textile industry was huge. They were worried about what would happen if they were cut off from Southern Cotton.
Support for abolition in the North was quite small. Abolitionist candidates got very small shares of the vote when the ran for office as a general rule. There was practically zero support for compensated emancipation if it meant they were going to have to pay any part of it. Even though New England had sold the slaves in the first place and the proceeds of that provided much of the start up capital for their manufacturers as well as universities.
They were against the spread of slavery for a variety of reasons ranging from wanting to diminish the political power of the Southern states which stood in the way of them being able to get economic policies that would favor them even more, to plain old fashioned racism (they were as against free blacks as they were slaves) to self interest. Since they did not have the economies of scale and could not compete on price with established manufacturers in Britain, they also could not offer wages that were as good. One of the big hooks they had to offer immigrant laborers was the prospect of obtaining their own land out west via homesteading. That was something European manufacturers could not offer their workforce. Thus it helped them obtain and retain for a few years at least, a workforce with which to compete against the European manufacturers.
The regions had highly specialized economies and diametrically opposing policy needs. That agriculture was saddled with paying for industrialization was not unusual. What was unusual was that it was so highly regionalized in the US. Add those different economic interests to cultural differences and different political cultures and you have a cocktail that makes separation of subjugation of one by the other inevitable.
Almost all wars boil down to money and economic interests. This one was no different.
Why do you suppose they felt high tariffs were needed on imported manufactured goods brought in from England, and not needed on agricultural goods (mostly cotton) produced in the South?
I wouldn't disagree with that. I only mentioned the abolitionists because a few wealthy Northerners did give money to John Brown and other anti-slavery activists, and neo-Confederates make much of that. They were atypical and not representative of the North as a whole or of wealthy bankers and industrialists.
But one word of caution: out and out abolitionists - people who'd be recognized as abolitionists today - were few, but there were plenty of ways Northerners could oppose the slaveholding interest and be labeled as "abolitionists" by pro-slavery extremists.
Since they did not have the economies of scale and could not compete on price with established manufacturers in Britain, they also could not offer wages that were as good.
By 1860, the US actually had more people (barely) than the UK, so the US domestic market was at least comparable to the British. I suspect that by 1860, Northern mill owners were competitive with British manufacturers. Competition meant they had to compete and weren't guaranteed that they'd come out on top, or even that they'd survive. The British could beat them out sometimes, since they had a head start, but I wouldn't say that US products were always more expensive or inferior.
That agriculture was saddled with paying for industrialization was not unusual.
My understanding is that farmers and manufacturers in the North recognized that they were working together. Trade between the cities and the countryside got canals and railroads built, and banks and warehouses and retail stores established.
Southerners had a political grievance -- they thought the Yankees threatened slavery -- and that made them resent Northern merchants and manufacturers. I'm not sure that middling and poor Southerners were as angry at Northerners at this time as wealthy and politically powerful planters were (obviously, that changed later and feelings were very different during the Populist Era).