Answer :

Northern success in the Siege of Vicksburg was important because the Union had a plan called the anaconda plan. The anaconda plan is when the Union planned to cut the Confederacy in half. By doing this, the Confederacy would not have any way to obtain (get) supplies (food, clothes, weaponry, etc.) When the siege of Vicksburg was a success, the Union was able to carry out the anaconda plan since the Mississippi River is in Vicksburg, and by taking over the Mississippi River, that split the Confederacy into two.