"Please don't mention my son," he asked the Marine Corps officer introducing him. Four days earlier, 2nd Lt. Robert M. Kelly , 29, had stepped on a land mine while leading a platoon of Marines in southern Afghanistan. He was killed instantly. At 6:10 a.m. on Nov. 9, Gen. Joseph F. Dunford Jr., one of Kelly's oldest and dearest friends, rang the doorbell at his home in the Washington Navy Yard. The instant Kelly saw Dunford, dressed in his service uniform, he knew Robert was dead. His wife, Karen, was still asleep. "I then did the most difficult thing I've...