I doubt God would be on the side of slave ownership. Of course, there is much more to the southern cause than slavery. I've read a fair amount of the conflict, and I believe the south lost for three interconnected reasons:
1. The north had the extreme upper hand when it came to manufacturing war material. This is the major reason.
2. The south made a mistaken assumption that cotton was king, and as such the European powers would intervene on the Confederacy's behalf. Being Europeans, they did not.
3. Despite having superior officers and fighting men, the south simply couldn't provide the private soldiers which the north could, primarily because the north was the destination of many immigrants who became Union soldiers. The south couldn't replace its casualties and the north could many times over. Grant knew this and prosecuted a war of attrition which the south couldn't possibly keep up with.
This is something i have discussed in length with a variety of people, including ministers and other intellectuals.
I, personally, do not agree with slavery.
However, I don't know if we can say God would be against it, as in several instances in the bible he ordered the Hebrews to take slaves from the people they conquered. And yes, the Hebrew word used means slaves, not servants.
It was common practice to take slaves from conquered peoples until the rise of "western" civilization after the dark ages.
Don't flame me now, I said I'm against slavery, I'm just throwing this thought out for consideration.