I agree, the war itself was fought over the issue of union and disunion.

However, I cannot imagine a secession over any other issue. No other issue had plagued the political landscape across the entire country for so many decades, with every other side issue becoming drawn into the politics of THAT main issue. Slavery was the bone of contention. Once the fight commenced the bone was no longer the main issue, winning was.

But the south could have won the fight, had they dropped the bone and used their teeth. They didn't, and their claws were not enough to win the battle all by themselves. The south held tight to the bone and lost the fight.