The uncomfortable truth of most wars is that the soldiers and citizens don't always have a clear idea of why the war is taking place.
Gallup polling during WWII showed a surprising number of people agreeing with the statement that they had "no clear idea what the war was about" even though they favored it.

The strongest argument that it was "states rights" actually comes from Lincoln himself. He was speaking sincerely when he said his first priority was to preserve the union, and if he could do it without freeing any slaves, he would.

Even then, you have to finish the sentence. "The civil war was fought over states' rights to legally enforce slavery."

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