You might want to consult /u/secessionisillegal's answer to If slavery was protected by the constitution via Dredd Scott (1857), and a constitutional amendment requires 75% of States and congress to approve, then how was secession the best response to Lincoln’s plan to ban slavery in the western territories? Lincoln didn’t have the votes.

/u/freedmenspatrol also answers Why can't the Civil War be called the "War of Northern Aggression," or a war about states' rights?

The answer to that question depends on when, and who, you ask. Although most historians would agree that slavery sits at the heart of the Civil War, there is certainly more nuance to the answer of exactly what role slavery played in Americans' motivations to fight the war.

First, for the Southern government and aristocracy (note that there was considerable overlap among these two groups) the Civil War was never not about slavery. Slavery was the engine that drove the Southern economy, and Southern tobacco and cotton planters' entire livelihoods were made on the backs of slaves; to abolish slavery was to completely upend their way of life. In the years leading up to the war, southerners grew increasingly anxious about the future of slavery as Congressional Republicans worked to limit its spread into the West. A great example of this anxiety lies in the Crittenden Compromise, a failed 1860 proposal to amend the US Constitution with explicit protections for slavery. It was proposed by Senator John Crittenden of Kentucky (hence the proposal's clever name) as a response to this anxiety; Crittenden and the proposal's supporters hoped that enshrining slavery permanently in the Constitution would put a stop to a secession movement that was rapidly picking up steam in the winter of 1860.

You can also look at Confederate Vice President Alexander Stephens's infamous "Cornerstone Speech," given in the wake of secession, in which he explicitly defends not only slavery, but white supremacy as the "Cornerstone" (again with the cleverly named documents!) of Confederate society. Stephens remarks "The [Confederate] constitution has put at rest, forever, all the agitating questions relating to our peculiar institution, African slavery as it exists amongst us – the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution." Stephens goes so far as to argue that the founding fathers' beliefs about universal equality were not just hypocritical, but completely misguided, and that the white race was inherently superior to others. His is probably one the clearest indicators that, for the South, the war was about white supremacy and slavery. (1)

For the Union, the answer becomes considerably muddier. There is often a misconception that all Unionists were abolitionists, but this is not the case. Even abolitionists specifically were not a cohesive group; they had a range of reasons for their beliefs, from free-soilers that opposed slavery for economic reasons, to radical Republicans that opposed it for moral ones. And in the border states, especially, there were a great deal of citizens that were reluctant both to leave the Union and to relinquish their slaves, leaving them torn between the two sides of the war. So the whole truth about what the war meant to northerners is often quite scattered.

Some historians, however, argue that white northern motivations evolved over time and that ultimately, most white northerners came to see the Civil War as a war against slavery, especially as they came increasingly into contact with slaves in the South. I specify "white northerners" here because for black northerners, the war was always about slavery.

Georgetown historian Chandra Manning has written an excellent book on exactly this issue, and she argues within that there existed a dichotomy between initial motivations for Union and Confederate soldiers; while Confederate soldiers felt compelled to protect their own way of life and/or their own individual liberties (which they felt were threatened by northern Republicans whether or not they owned slaves), Union motivation was borne instead of patriotism and a belief that the US government stood for more global ideals of liberty. So, while few white northerners enlisted with abolition in mind, you could argue that their motivations for fighting the war evolved alongside their government's in that what began as a war to preserve the Union increasingly became a war against slavery. Manning notes that as the tides shifted in favor of the Union in the latter half of the war, many northerners jumped on an opportunity not just to preserve the Union, but to reform it. (2)

References:

(1) Stephens, Alexander H. "Cornerstone Speech." Savannah, Georgia. 22 March, 1861.

(2) Manning, Chandra. What This Cruel War Was Over. (New York: Random House, 2007).