he Siege of Vicksburg (May 18 July 4 1863) was the final major military action in the Vicksburg Campaign of the American Civil War. In a series of maneuvers Union Maj. Gen. Ulysses S. Grant and his Army of the Tennessee crossed the Mississippi River and drove the Confederate Army of Vicksburg led by Lt. Gen. John C. Pemberton into the defensive lines surrounding the fortress city of Vicksburg Mississippi.