What was one reason why the US fought the civil war?

After the War of Independence the 13 states agreed to a fairly weak federal system where a central government was superior to the state government. Gradually the central government got stronger, writing a constitution in secret, and attempted to impose its will (dominated by the industrial north) on the states. The southern states, culturally very different from the northern states, objected to the increase in central control and eventually, against the will of the north, seceded.