Question

Why was slavery the cause of the civil war

Answer 1

The North and South were arguing over slavery. The South wanted slaves but the North didn't.

Answer 2

Slavery was the cause of the civil war, because the Northern side did not believe that enslaving people for work with no pay was wrong, while the Southernors believed that it was in their right that they can own slaves & use them in whatever way they wanted to.

In this disagreement they started a war (along with other factors)