The city of Vicksburg, Mississippi, was the last major port on the Mississippi River held by the South. If the North could take Vicksburg, the Confederacy would be cut into two pieces: the east and the west. General Ulysses S. Grant of the Union army was convinced that the key to winning the war was the control of the Mississippi River. Taking Vicksburg would be the last step needed to achieve that goal. His first two attempts were failures, but Grant didn't give up.
Why was Vicksburg considered "the key" to winning the Civil War?