Why do you want to know? This sounds like an assignment for school. If so, that's fine, but surely you've been supplied with a textbook which you could use to help answer this question. My point is that any college or high school textbook on American history should be sufficient. The biggest problem in answering this question is that it requires a comprehensive understanding of US history, in that you need to assess the significance of slavery as compared with other factors, whatever they might be. That means a book about slavery itself will not be enough, and a more general work like a textbook might be the way to go.

I hope you've got some time to answer this question because you've got some deep thinking to do. I'm happy to provide additional assistance, but if this is an assignment, more details about its parameters would be helpful.