Certainly the issue of slavery was a big part of the Civil War equation, but it is a bit far-fetched to imagine tens and hundreds of thousands of northern white men leaving their homes and families to put their lives on the line for years on end, because they knew the cause was just, and they were willing to die for the freedom of their black brothers and sisters in the South.
Give me a break.
In the same vein, I can't really imagine tens and hundreds of thousands of Southern boys going off to fight and die for the right of some rich slave owners to own slaves, either. These people who went off to fight didn't own slaves. Or even a winter coat. Some of the generals and politicians did, but not the boys with no shoes, for whom it was probably rare to even interact with slaves. But these men went off willing to fight, just as the Northern men did. Why, then?
Other people say the Civil War was fought over the principle of what came to be known as "States Rights" - the contention that the Federal Government was usurping the constitutional sovereignty of individual states more and more with each passing year. Many people in the South felt the North was bullying them with unfair laws and taxes, and generally conspiring to screw them over, and that it was getting progressively worse with every passing year. These people say (with some truth, frankly) that the situation finally became intolerable. Just as certain British laws and taxes had become intolerable to their ancestors, four-score and seven years earlier, so, too, had the Federal Government - the NORTHERN government - become a hated symbol of oppression in the South.
The Civil War didn't resolve that way of thinking, of course; it is still very prevalent in our politics even today.
Were either of these things the reason the U.S. had a civil war?
If we take the word of the man who was the U.S. President during the Civil War, the reason the war was fought was "to preserve the Union" and nothing else. To quote him:
"My paramount object in this struggle is to save the Union, and is not either to save or to destroy slavery. If I could save the Union without freeing any slave I would do it, and if I could save it by freeing all the slaves I would do it; and if I could save it by freeing some and leaving others alone I would also do that. What I do about slavery, and the colored race, I do because I believe it helps to save the Union; and what I forbear, I forbear because I do not believe it would help to save the Union. I shall do less whenever I shall believe what I am doing hurts the cause, and I shall do more whenever I shall believe doing more will help the cause. I shall try to correct errors when shown to be errors; and I shall adopt new views so fast as they shall appear to be true views."
And then he added:
"I have here stated my purpose according to my view of official duty; and I intend no modification of my oft-expressed personal wish that all men everywhere could be free."
This was in August of 1862. A few weeks later he signed the Emancipation Proclamation. Apparently he thought freeing the slaves would injure the South's war effort. So did many others. Of course the Proclamation meant nothing until the North won the war.
What do I think personally was the reason for fighting the Civil War? I've tried to read books on both sides and try to understand both sides over the years. My inclination is to take Lincoln's statement at face value and say the Civil War was fought to preserve the Union.
The only minor tidbit, though, is that the U.S. Constitution doesn't forbid states from leaving the Union. It only says Congress shall prescribe by law the manner in which new states are admitted. And all Congress required was a blood test and a 3-day waiting period. (Actually, it requires most of the other states to agree to let the new state join the club.) Neither says anything about having to stay 'til death do them part. I guess no one contemplated that, once admitted to Heavenly Union, no state would ever contemplate giving up such divine bliss.
Legally, then, the South was right.
Moral? I suppose it would be that "being legally right" doesn't always mean you get to do it unmolested. The North had more than a little invested in the South over the years - not just money but many other things - and felt the country - as a whole - now sort of "owned" the South, in a manner of speaking. In the larger scheme of things, everybody owned everybody else, and nobody was just going to be allowed to take their ball and go home. Sometimes, even though it is your ball, someone bigger and stronger sometimes comes along and forces you to stay in the game instead of going your own way with your ball. Or your slaves, either, as far as that goes.
What do YOU think the cause of the Civil War was?