Major General Jesse Reno, Commander of the 9th Corps of the Union Army of the Potomac, was killed three days before the battle of Antietam.  But because General Reno was a Corps Commander in General George B. McClellan's Army, and because McClellan has been reported as the villain of the war (and unjustly so) Reno has been a forgotten man, in large measure, because of that relationship with George McClellan.  

What if he had not been killed, and had been at Antietam?  Would the Union Army have destroyed Robert E. Lee's Army of Northern Virginia?   Would the war have ended had Jesse Reno been in charge of the 9th Corps at Antietam, and not Ambrose Burnside?

For 160 years this question has been obscured because historians have been preoccupied with other things.  Mainly, in their collective attacks on George McClellan.  And because historians negative obsession with George McClellan, many of the things that happened, and the men involved in the great battles of the Civil War, have been mis-reported, obscured or forgotten.  Historians are now reassessing these narratives and histories, and the truth is starting to come out.