Slavery was the bone over which the two sides fought, but the real cause was a House divided, as Lincoln pointed out. Each side had too many firebrands who thought that the country had to be either slave or free. It was a clash of absolutes. There were moderates in both North and South, the Southern loyalists in Eastern Tennessee, the “Copperheads,” of NYC. But with Lincoln determined to hold the two disparate parts together by armed force, and without a decisive victory by either side, the passions boiled over.
In other words slavery was the linchpin and other issues sprung up around that principal one. To say the real cause was “a house divided” - well OK but what precisely were they divided over?
In some sense - it was like a messy divorce - the fire had to get started somehow but once it did there were all sorts of fuel to keep it going.