To expand, I sometimes have discussions about history with my mother, who was born and raised in the post-WWII south. I've, with help from this subreddit, explained that the institution of slavery was obviously the cause of disunion, as she was taught the entire war was over a heated debate over state's rights that turned bloody.
I am aware that, until the latter half of the war, Union soldiers were motivated to preserve the Union, and not to end slavery. However, I'm not so sure about why Confederate soldiers fought. She maintains that their personal motivation was State's Rights and as such, it is accurate to say the war was fought over state's rights, even if it was started by slavery. Can anyone enlighten our discussion?
Edit: Could anyone provide a good source of primary documents of soldiers at the time? My mother is convinced that, do to education level at the time, "Preserve the Union" was too complicated for most people to understand enough to the point they'd ever consider fighting. I'm pretty sure that's bunk.
Edit again: Just had a follow up with my mother, she still has her question. I'll let her ask it:
Was the common soldier from the north able to be told 'you are fighting to preserve the union' and be willing to pick up a gun without being conscripted? And was the common soldier in the south, ie a sharecropper or dirt farmer willing to pick up a gun to preserve state's rights? Were either ways reasonable ways to draw volunteers? Would those statements have meant anything to the 'common man' of the day? If not, what was used to get the average volunteer fighting?
The statement that the war was fought over state rights is an old way of thinking about the war. It wasn't. If it was fought over state's rights then why did the South push so hard to violate Northern state's rights with the fugitive slave act? The state's rights argument is an old, sympathetic, southern apologist style label that is given for southerners fighting the war. They might have even believed that themselves, but really they were trying to protect their "Peculiar institution."
I would highly recommend you look at Edward Ayers's the Valley of the Shadow that has a wealth of primary sources, from both the North and South. It's a focus on two communities that are very similar and really not that far from each other in the Shenendoah valley. Here's the link: http://valley.lib.virginia.edu/
I would highly recommend James McPherson's What They Fought For 1861-1865 in which he analyzes the letters and diaries of nearly a thousand Civil War soldiers, both Union and Confederate. He does a great job of breaking down the different motivations of these soldiers and as I mentioned above uses letters to home and diary entries as primary source material.
Motives varied widely in the North. The English stock that had been around longer cared more about the survival of the nation, while Germans and the Irish were heavily abolitionist and volunteered to end slavery.
Soldiers fought in the Civil War for many reasons. Avoid anything that tries to simplify it to one thing.
Slavery was the primary motivation for secession, so that can't be ignored, but there were still plenty of different perspectives. I know of cities in the deep South that argued against secession and war. Plenty of Southerners were conscripts or joined up because they expected to be conscripted. Some fought for regional pride. Others fought to preserve their status as being superior to the black man. And then there were plenty of people who owned slaves, wanted to own slaves, or had some other reason to want to see the continuation of slavery.
The flip side is that many Union soldiers didn't care one bit about black freedom. Some cared about preserving the nation. Others were conscripted. And then there were serious abolitionists and even black soldiers (who weren't necessarily treated very well by the Union).
I'd recommend reading the sources posted by some other people already.