Follow up question...

Don't many of the founding documents for the Confederacy directly state that the cause of the secession was slavery? Makes me wonder why none of these Confederacy apologists ever mention that...

The war began over the issue of slavery. But the progression of the war itself was due to complex reasons, as is typical for wars. Once you get into a fight the reason you keep fighting is not the reason you started fighting, it's because you want to live and protect yourself. That's a big reason why it's so easy to point to any major war and say it had complicated causes.

The cause of the war was the secession of the Southern states. And the Southern states seceded over the cause of preserving the institution of slavery. The speeches, declarations from Southern politicians at the time and the contents of the new constitutions of the member states of the CSA as well as the CSA itself (typically very similar to others but with the notable exception of calling out and specially protecting the institution of slavery) make that abundantly clear.

One might wonder why the South seceded over the election of Lincoln since he did not precisely run on a campaign of abolition. It's important to understand that the South had always had a firm grasp on power in the US since its founding, and were always able to control the issue of slavery sufficiently well to ensure its continued longevity for the foreseeable future. Lincoln's election wasn't obviously the immediate end of slavery for the US, but it was an obvious immediate end to the political power of the South. Lincoln spoke about the issue of slavery openly and plainly, and avoided tap dancing around the issue the way most other politicians would. He hauled the issue out in the open and made plain that something would have to be done about it sooner rather than later, and that "something" was unlikely to be indefinite toleration of the practice at a federal level. That alone was too much for slave holders, who bent all of their political power toward preventing his election, and failed. Lincoln wasn't even on the ballot in many Southern states, but it didn't matter, he still won.

Slavery supporters in the South saw the writing on the wall, they saw themselves vilified, saw the institution of slavery demonized, and they saw those doing the vilification and demonization coming into power. Those with foresight could see a legal end to slavery as expansion of the Union would increasingly add non-slave states and then ultimately the balance of power in the Senate would shift enough to merely outlaw slavery with a federal law (or to work through the process of amending the constitution). Those without foresight were merely aggrieved to see what they perceived as an enemy assume the presidency. That was enough to leverage the long-held differences between the South and the rest of the Union to drive for secession in order to preserve slavery, and that triggered everything that came after.