Read any state’s declaration of secession. Slavery was the direct cause of the war.
Not by the North to end it, by the South to preserve it.
> The General Government, as the common agent, passed laws to carry into effect these stipulations of the States. For many years these laws were executed. But an increasing hostility on the part of the non-slaveholding States to the institution of slavery, has led to a disregard of their obligations, and the laws of the General Government have ceased to effect the objects of the Constitution.