Slavery caused the American Civil War. Of course, it wasn’t the only reason war came, and most soldiers, either Union or Confederate, fought for their own personal reasons, but slavery was ultimately behind the fundamental rift between the states.
Economically, slavery played a significant role in producing wealth in the Southern states. Unlike the Northern states, the Southern states were largely agricultural. They used millions of slaves for manual labor.
For the Northerners, it was a case of slave labor versus free labor. What would happened if “slave power” expanded its grip over the entire nation? They certainly didn't want to find out.
Examining the various acts that were passed before the war also demonstrates the link between slavery and the Civil War.
For example, the Compromise of 1850 consisted of a package of five bills. The most notable was the Fugitive Slave Act. This law required individuals, including judicial officials, to aid in capturing escaped slaves and return them to their owners. The ‘escaped slave’ could be a freedman, but it could rarely be determined because no court trial was needed.
Finally, when President Lincoln was elected, he took steps to abolish the practice of slavery from expanding in the territories. This was the last straw in the Southern states' drive to secession.
And, in any event, the Emancipation Proclamation and the Thirteenth Amendment rendered the issue moot. The direct consequence of the war was the end of slavery.