In the spring of 1864, Robert E. Lee faced a new adversary: Lieutenant General Ulysses S. Grant. Named commander of all Union armies in March, Grant quickly went on the offensive against Lee in Virginia. On May 4, Grant's army struck hard across the Rapidan River into north-central Virginia, with Lee's army contesting every mile. They fought for forty days until, finally, the Union army crossed the James River and began the siege of Petersburg.The campaign cost more than 100,000 men,the largest loss the war had seen. While Grant lost nearly twice as many men as Lee did, he could replace them. Lee could not, and he would never again mount a major offensive. Lee's surrender at Appomattox less than a year later was the denouement of the drama begun in those crucial forty days.