What were the important issues that divided the North and South prior to the Civil War? At what point do you believe war became inevitable?

What were the important issues that divided the North and South prior to the Civil War? At what point do you believe war became inevitable? When war broke out, what was the Union's official policy towards the institution of slavery? Why did it change? What important executive and legislative measures were taken between 1861 and 1877 to extend greater rights to blacks? How did the Confederacy/South respond?