Was the civil war over slavery?

Not really.

Let's start with where it began: Fort Sumter, which was used by the U.S. as a tariff collection port. It's not a coincidence this was the location for the first battle of the Civil War.

Throughout the 1800s, the federal government was mostly funded through tariffs. Because of the agricultural economy of the South, and its relatively high amount of exports, the southern states disproportionately funded the federal government. At the same time, the manufacturing industry of the northern states benefited from protectionist trade policies.

The central theme of Lincoln's campaign was economic nationalism. Which meant support for the recently passed Morrill Tariff (Correction: pointed out below, the tariff had not yet passed. It was supported by Lincoln, is my main point here), and possibly further increases in taxes on southern exports.

While Lincoln was anti-slavery, he was not an abolitionist. He made it clear that without southern secession, he had no plans to interfere with slavery in the south:

"I have said a hundred times, and I have now no inclination to take it back, that I believe there is no right, and ought to be no inclination in the people of the free States to enter into the slave States, and interfere with the question of slavery at all." - Honest Abe

It's well known that the Emancipation Proclamation didn't come until 1863, when it was needed as a political move (swayed public opinion in the UK from favoring the Confederacy to favoring the Union, and also opened up the possibility of having freed slaves fight against the South). Of note, it only ended slavery in the states in rebellion, not in the border states that remained in the Union.

Common objections

1. "But political leaders in the South said themselves that the war was over slavery" -- Yes, some did. Others emphasized the importance of state sovereignty. Regardless, this isn't really a good measure for determining the actual causes of war. Most Southerners were racist. Most northerners were racist. Playing up Lincoln as a supreme abolitionist was a good way to drum up support from the average citizen (on both sides of the Mason-Dixon) that may not have had as much stake, or interest, in a debate over trade policy.


2. "Lincoln may not have planned to end slavery in the South, but he did want to prevent the spread of slavery west."
-- This is a valid point and why my answer to the original question is "not really." I won't say that slavery played no role at all. There was important political power at stake for slave owners here. Whether new western states became free or slave states would have definitely played a role in the longevity of the institution of slavery.

However this was a less pressing issue than the tariff matter. Would this have caused secession at a later point in time? Possibly. I find it really unlikely that the pure possibility of future states outlawing slavery was the driving force behind secession.