Error: your use of the word "the". See my post above.
No error, slavery was the primary cause for the war.
In any war there are always various causes, but there is one overriding that ignites the fire.
Slavery was both the immediate cause and the primary cause.
The idea that slavery was not the issue of the war came after the war when dishonest confederates wanted to pretend that they were not fighting to uphold an immoral system.
Stephen's speech shows otherwise.