So What If They Were Fighting for Slavery?

This question was originally posed to Corey Meyer in my blog comments, but I'd find it interesting to see anyone's answer -- floggers, their followers, random visitors... I'm interested in people's personal view, not something copied from a book...

Let's say for a moment, hypothetically, that the South was fighting to preserve slavery. What is the significance of that, in your opinion?

If you say, "They were fighting to preserve slavery," and somebody says, "So what?" how do you answer that?

Awaiting answers in the comments, folks.