The Siege of Vicksburg was the final major action in the Vicksburg Campaign of the American Civil War. In a series of manoeuvres, Union Maj. Gen. Ulysses S. Grant and his Army of the Tennessee crossed the Mississippi River and drove the Confederate army of Lt. Gen. John C. Pemberton into defensive lines surrounding the fortress city of Vicksburg, Mississippi. After two assaults (May 19 and May 22) against the Confederate fortifications were repulsed with heavy casualties, Grant besieged the city from May 25 to July 4, 1863, until it surrendered, yielding command of the Mississippi River to the Union. The Confederate surrender at Vicksburg is sometimes considered, when combined with Robert E. Lee's defeat at Gettysburg the previous day, the turning point of the war.