Mississippi gives what as it main reason for succeeding for the union?

Mississippi's declaration in defence of the institution was slavery which became the ultimate reason why the state seceded from the union.

Explanation:

Before the Civil War, slavery increased immensely in Mississippi with black slave population much more than white slave population. The state's economy was managed by cotton production which depended heavily on slave labour.

The black slaves were treated with much inequality and were forced to the cruelty. US stated that the slavery of the black race is saddening in nature which made Mississippi secede from the union. Hence, it was only slavery that made Mississippi secede from the union.