I know that the Southern states had these documents stating their independence from the U.S. because they believed that the U.S. didn't realize the importance of slavery. Or at least that's what I picked up from the 'Declaration of Causes of Seceding States'.

But, if I go online and search up some discussions about these topics, they seem very heated and it just seems to drag today's politics into it so I just wanted to ask you historians out there, what was it? Was the North really concerned about slavery first and foremost or was slavery one of the reasons they went to war? Also, please show some sources to back up your claims so I can go and research those myself as well. Thank you!

Edit: Already knew it. 10 minutes in, 50% upvoted, only 2 comments, none of them have sources...

Edit 2: Thank you, keyilan (they're a mod so I'm not sure whether I should tag; came from discord, mods don't like to be pinged much) for providing an extended and detailed answer link.