What was the US civil war about?
The US civil war had a complex start but the main aim was the preservation of slavery for the confederacy and the elimination of slavery for the union. Slavery was the principal political issue between the parties. 
There was also an aspect of the confederacy wanting to secede from the Union to preserve slavery. The union did not want the slave stares to secede. After war broke out, it is interesting to note, no foreign government recognized the confederate government. 
By the mid 1800s, the US had two distinct regions: free states with fast growing economies based on family farming, manufacturing, mining, business and transporting goods. The slave states were based on a plantation system and the only fast growth was in texas. These states were largely rural with few large cities and little industry except in close proximity to free states.