Except that slavery cannot be divorced from economic growth. Slavery had to be resolved to get immigration going again and to cause European investors, mainly in Britain, to have confidence in the US economy. Slavery was a yoke around the neck of the US, which made a few people rich and gave them great political power. If a politician wanted to curtail and end slavery, he also wanted to end the slave owners' alliance from controlling US economic growth. Missouri, Iowa, Kansas and Nebraska took off after the US Civil. The main issue was resolved and growth mushroomed. The same thing happened in Texas, after a brief delay.Zooming out for a moment, I think this thread provides a really interesting example of what makes studying history so difficult. Much as we want to hammer people’s motivations down to specific things, humans act for nebulous and contradictory reasons. Sometimes, esoteric ideas like “freedom” or “Union” really can motivate people to act. Other times, it takes concrete things like money to push action. Though the trope is that it all comes back to the money, humans have also proven to sometimes be driven solely or mostly by ideas.
This relates to different schools of thought within the historical community. Charles and Mary Beard are famous examples of the more economically focused school of history/progressive historiography. This interpretation of history has since fallen out of favor in most cases. The Beards in their discussion of the civil war downplayed slavery, abolitionism, and morality issues in favor of economic ones.
Other historians have shifted focus to those ideas of slavery and morality as driving motivators. Similarly, some historians have viewed the role of religion as a motivator for action.
All of this is to say that looking for motivating factors is difficult and it’s important to not approach from a preconceived endpoint and then find the evidence to back up the idea. As many have pointed out, the tariff argument does not square with the evidence, and consequently examinations of Union war aims must shift accordingly.
It also speaks to a larger idea that history is not a static idea but an ever-changing subject. But that’s for another time.