There was another thing, which slavery was the underscoring issue to highlight and bring to a head, and that was: Who has final say, the State legislature(s), or Washington.

The question was brought to a boil by the question of slavery, but really, the problem had been kicking around for quite some time...

...And like almost every other situation in history where there were two powers and a question of who has final authority, it led to civil war.

For example, the British civil was was over the question of who would have final authority, Parliament or the Crown. IN Britain's case, Parliament won... but just barely, and even after they won, the de-facto Prime Minister Cromwell had to throttle back and lock down the power of Parliament and run England as if a Crown still had final say because the elected reps hadn't figured out how to govern as a body.

Prior to the US civil war, American diplomats to other nations used to refer to policy statements as coming from "WE United States want ...", but after the US civil war, diplomats would say "THE United States wants..."

No I don't think Slavery was the only reason. Most wars are fought for power and this was no exception. People got rich off of that war and some of those same people helped whip up the frenzy which launched the war.