The dominant view of the Civil War, at least among historians, is that the Civil War was fought over slavery. After all, if there had been no slavery, there would have been no war. True, but that doesn't mean the war was fought over slavery. Similarly, if there had been no Kuwaiti oil fields, there would have been no Persian Gulf War in 1991, but of course that doesn't prove that that war was fought over oil fields--it was fought over Iraq's invasion and occupation of Kuwait.
Most historians also claim that Southern secession was all about slavery and that therefore the Civil War was fought over slavery. But, for one thing, leaving aside the fact that secession and the war were two different events, the claim that Southern secession was all about slavery is demonstrably incorrect. The four Upper South states rejected secession when it was based largely on slavery and the tariff. They felt that the Deep South states had valid complaints relating to slavery and the tariff, but they did not feel that those complaints justified secession. The Upper South states decided to leave the Union only after Lincoln made it clear that he was going to invade the South. Until then, pro-Union sentiment was strong and arguably gaining ground in the Upper South.
As for why the Deep South states seceded, the record is clear that there were two major reasons: slavery and the tariff, in that order. Slavery was the main reason the Deep South seceded, but the tariff was also a major reason, which is why four of the seven Deep South states mentioned or alluded to the tariff in some of their official secession documents. Southern newspapers printed numerous editorials on the tariff and cited the Republican-backed Morrill Tariff as a major reason to leave the Union.
And it should be pointed out that the debate over slavery was not over whether slavery should continue but over whether it should be allowed to expand into the western territories. Lincoln made it very clear that he had no interest in disturbing slavery where it already existed; he just didn't want it to spread beyond its current boundaries. Most Republicans held this view.
In fact, Lincoln supported the Corwin Amendment, which would have permanently prevented the federal government from abolishing slavery. Lincoln not only pushed for the amendment behind the scenes, but he mentioned his support for it in his first inaugural address.
It is interesting to consider that when Lincoln sent the armed federal naval convoy to Fort Sumter--the convoy that provoked the Confederacy to launch an attack on the fort--there were more slave states in the Union than there were in the Confederacy.
Moreover, when the war began, the U.S. Congress, by a huge majority, passed a resolution that avowed that the war was not being fought to end slavery but only to restore the Union. It was over a year later, and only after Union casualties began to greatly exceed expectations, that Lincoln decided to include abolition as a war objective.
Furthermore, by late 1864, most Confederate leaders, and apparently a substantial majority of Southern citizens, were prepared to end slavery in order to preserve Southern independence. In early 1865 the Confederacy began to move toward gradual emancipation. This shows that, when push came to shove, independence was more important to the South than the continuation of slavery.
So what was the Civil War fought over? Answer: Independence--Southern independence, to be exact. The South wanted to leave the Union and form an independent nation. The North invaded (over the opposition of many Northern citizens) in order to force the South to rejoin the Union. Independence was the central issue, the main point of contention. Everything else was secondary. If the South had not claimed independence, there would have been no war, and slavery would have continued for a few more decades and then died a natural death.
If the Confederacy had announced at its founding convention that in one month it would begin a program of gradual emancipation patterned after previous Northern emancipation programs, would Lincoln have abandoned Fort Sumter? Would the Republicans have agreed to accept the Confederate peace proposal? Would they have recognized the Confederacy? Would they have at least been willing to peacefully coexist with the Confederacy? Any serious student of the war knows that the answer to these questions is no.That's because the central issue of the war was Southern independence, not slavery.