A civil war is a war between a government and faction of  political dissidents who attempting to overthrow the existing government. The so called American Civil War was not a civil war. The 11 southern states had seceded from the Union and had no desire overthrow the government of the United States, they simply wanted to secede from the Union.

The Tenth Amendment

Any power not granted to the United States that is not prohibited to the states is reserved to the states and to the people. 

Since the power to secede was not prohibited to the states, it is a power reserved to the states.

The states that seceded believed that they were being unfairly taxed. Even though the south represented only 30% of the population they were paying over 80% of the taxes.