The Southern states (at least the initial ones) seceded over slavery, yes. The war, however, was over the states' right to secede (and the adverse impact of the secession on the country as a whole — particularly economically) far more than it was slavery, itself.
Its being about slavery was just the story offered to the polity in order to sell them on having a war at all.
> Its being about slavery was just the story offered to the polity in order to sell them on a having war at all.
The Union never claimed the war was about slavery (which would have been counterproductive, since the North contained slave states), but instead consistently said the war was about preserving the Union.
So, no, it wasn't a story offered to the polity to sell them on having a war. Because it wasn't the story offered to the polity at all.
Every grade school class I've heard of has taught that Lincoln was elected on a platform of preserving the union, that the South seceded over slavery, that the north -- including slave states -- fought to preserve the union, and that ending slavery was an effect of, not the motive for, the war.
Its being about slavery was just the story offered to the polity in order to sell them on having a war at all.