The holding of slaves was a contentious issue from the beginning of the Republic--the founding fathers were compelled to move forward without resolving the issue--there was an uneasy balance of power between slave-holding and non-slave holding states, the proposed annexation of Misouri in 1820 nearly tore the Union apart--

When the Misouri Compromise(which basically disallowed slavery above 36°30') was repealed by the Kansas Nebraska Act in 1854(which allowed territories to chose whether they would enter the Union as slave or Free states)the slave and free states were at each others throats once more--open warfare broke out in Kansas, where voters chose twice to be a free state, but were denied it by the Democrats in Washington, and the Republican Party was formed with the avowed goal of eliminating slavery entirely.

The first slave holding states to leave the Union did so in disgust after the election of Lincoln, a Republican, who tried (unsuccessfully) to softpedal his Party's objectives. The rest left when Lincoln called for the states to provide a militia to respond to the Fort Sumpter situation, which they regarded as an attempt at coercion--

To be fair(which I am, sometimes;-))in the North, actions tended to be precipitated by anti-slavery feelings, the states in the South had strong feeling that their sovereignty had been and would continue to be violated by the Northern states. Most Southerners did not keep slaves, and often, if not generally, didn't believe that it was right to keep them, but they also felt that it was their legal right to deal with the problem, and not that of the Union.

It would be wrong to say that the war was about whether slavery was acceptible or not, but the question of slavery was the fundamental aggravation in the conflict.

Without wanting to seem like I am backtracking on my original statement(and, at the same time, doing it), I will say that issues, whether they are slavery, reproductive rights, prohibition, free soil, BiMetalism, or any of the other things Americans have ended up at each others throats about, are merely aggravations of conflicts between competing powers and interests that we've never figured out how to reconcile.