The correct answer is: They conducted a siege on the city of Vicksburg. This city was key to the defense of the Mississippi River. After some failures, Grant thought of a very skilful move to take the city from the east, mobilizing his army into a fleet commanded by Admiral David Dixon Porter. They besieged the city of Vicksburg for 47 days and finally had the victory. Grant and Porter had a remarkable relationship both professionally and personally, was a couple with a high impact on the outcome of the war.