Gaining control of the missisipi river during the civil war would allow the north to

It'd allow the North to control goods going down the river to the South. Doing this, the North could cut off food or weapon supply to "starve out" the other side. The North could also use the Mississippi for transporting their own troops into battle.

under god i have placed my chiest strentgh and the safe guard to my loyal subjects

definitely a for sure. ill see and update if i get the answer