I know this is one of the most worn-out historical discussions of all time, and it's probably one of the most disliked. The whole topic is plagued with Lost Causers, apologists, myths, and so on. I am, however, from the American Southeast, and I love reading as much history as I can get my hands on, so this subject is by far one of my favorites to read about. Because of where I was brought up, my perception is no doubt biased, so I will refrain from imposing my bias as best as I can.
The most common and accepted argument is that the Civil War was fought over the issue of slavery. A less accepted answer is states rights, which is often met with the retort "a state's right to own slaves". For a long time this is what I believed, as it was widely taught in public schools where I live. I acknowledge that the Civil War was fought over slavery just because it is what is most accepted by the population.
The issue surrounding slavery was most certainly the driving force behind secession. I seem to recall reading documents written by the state governments themselves that the threat of abolition was one of, if not the main reason they seceded.
But the secession of the southern states and the outbreak of war are different, at least as far as I understand it.
I've never came across anything that even vaguely suggests Federal armies were being raised to free slaves in the Southern states. This is even more far-fetched considering slave states were still in the Union (although I am aware Lincoln could not afford to abolish slavery in these critically important border states so early in the war). Similarly, there's nothing suggesting the Confederacy mobilized her armies to protect against Federal attempts to free slaves.
My choice book on the subject has been Battle Cry of Freedom, by James McPherson. After reading through the first few chapters I began to formulate a new perspective on why the Civil War was being fought. The Federal government refused to acknowledge the supposed 'Confederate States of America', and refused to hand over assets and garrisons. Although the South wanted to refrain from attacking, so-called 'fire-eaters' urged an attack on Fort Sumter against Beauregard's orders. This hostile action sparked conflict, prompting Abraham Lincoln to raise forces with which to invade the South.
As for why the Civil War was fought, it appears to me that Abraham Lincoln mobilized his forces to subdue the southern states and bring them back into the Union. The Confederacy, on the other hand, was fighting to prevent that. Neither belligerent seemed to have war goals concerning slavery.
President Lincoln brought abolition to the table after Gettysburg in 1863, and afterwards freeing slaves did become a Federal military objective. But the war had started nearly two years prior, in 1861, for completely unrelated reasons. It doesn't seem logical to claim the Civil War started over an issue that only became a military objective midway through the conflict.
I want to apologize if my post seems biased; I am no historian, nor an expert. Perhaps I have dissected the topic to fit my own perspectives - a futile attempt to cut slavery out of the picture. From a Historian's point of view, why did the American Civil War start, what sparked open conflict, and what prompted the Union and Confederate armies to attack each other?
One of the key things to consider is the political position of the newly elected Republican Party before the war ever started. A majority of Northern voters elected Lincoln on a platform of ending the extension of slavery, in order to place it on an ultimate course toward extinction. During the 1850s, acquiescence to the Southern Slave-Power had allowed for an extensive Fugitive Slave Act, a repeal of the Missouri Compromise, and the Dred Scott decision. By the end of the decade it appeared as if the institution might spread, not just to the Western Territories, but to all the States. Lincoln claims as much in his famous “House Divided” speech. He explains that his belief is that the Union would not endure permanently half slave and half free, and that it would become all one thing or all the other.
Either the opponents of slavery, will arrest the further spread of it, and place it where the public mind shall rest in the belief that it is in the course of ultimate extinction; or its advocates will push it forward, till it shall become alike lawful in all the States, old as well as new -- North as well as South.
Have we no tendency to the latter condition?
It is on this principle that the Republicans came to power. Arrest the spread of slavery and it will start to die off one way or another. Treat it as a bad thing that needs to be cured through careful, gradual measures. This is of course something that Southerners saw as well. So they viewed this Northern position as a mortal threat to their society. On top of that, they believed the North had become radicalized toward abolition, so the John Brown types that would incite slave rebellion were only to get more numerous, powerful and dangerous. And the new Federal Administration would do little to nothing to stop it, and would perhaps even support it unofficially.
After Lincoln’s election and the subsequent tide of secession, calls were made for Lincoln and the Republicans to walk back their hard line stance against slavery, in the spirit of Union. But Lincoln refused. There would be no more bowing down to the Slave power. He won the election fair and square on those principles, and to back out now would be an absolute insult to Democracy and the will of the people. Being bullied to accept a policy of appeasement was perhaps just as dangerous as accepting secession. We are one nation that can follow the will of the people, and the will of the people chose to steer the country toward a policy of treating slavery as a wrong.
That being said, as most already know, the Republicans stopped short of calling for an immediate end to slavery where it existed. The only real feasible way of dealing with it that was largely accepted was through this gradual, natural process. And Northern Democrats were still largely against any hard line stance against slavery. Slavery was implicitly allowed for in the Constitution. So of course, Lincoln could not wage an outright war to end slavery in 1861. It would have all but ensured support for saving the Country would dry up. And it would give credibility to all the accusations that the Republicans were simply waging a radical abolition war-John Brown on a grand, Federally sanctioned scale. So the immediate goal was to quickly end the rebellion and proceed as usual through peaceful political means. This is why Lincoln can claim in his First inaugural that he has no intention of ending slavery where it already exists (something found on any Lost-Cause bingo card), yet in the same speech claim that:
One section of our country believes slavery is right and ought to be extended, while the other believes it is wrong and ought not to be extended. This is the only substantial dispute.
The slavery question does not simply spring up out of a vacuum mid-war. Nor was it a minor issue that became a major one. It was the issue that divided the Country. The Political conflict was being waged and neither side was backing down from it. Moderates that were plenty in 1860 became forced to take sides for or against slavery. Because that was the root of the issue. The Lincoln Administration took steps to attack that more directly, long before Gettysburg (I believe you are probably thinking of Antietam here). As early as the summer of 1861, they took gradual steps starting with the First Confiscation Act to attack it directly. By the 2nd second year of the war, in the summer of ‘62 Lincoln had already decided that a “soft war” to end the rebellion and resume the political struggle was impossible. The Emancipation Proclamation was drafted that summer (a year before Gettysburg) and issued as a preliminary document in the fall, going into affect January 1st 1863. This turned the war into an outright revolution. And from that point on, the North would explicitly fight to free slaves.
So in short, the entire conflict of the era-political and military-is over the future of slavery. The war was not explicitly fought for abolition at the outset, because the military goal was to end the rebellion and resume the political fight to put slavery on an ultimate course toward extinction. In 1861 it was not thought to be legal, or plausible that slavery could be ended through military action. The war changed that in less than 2 years. It is often claimed that Lincoln only cared about the Union and didn’t care about slavery. But the reality is that he welcomed a fight against the slave power rather than let the Union with slavery everywhere take hold. He would rather risk the Union entirely than bow down to those that would let it become something not worth fighting for at all.
It started because of slavery, and the immediate war aims of the Confederate states were to preserve and extend slavery. This is true even if the Union armies were not initially on an antislavery crusade. For much more about this, feel free to visit this section of our FAQ:
https://www.reddit.com/r/AskHistorians/wiki/civilwar#wiki_the_civil_war_and_slavery