The Regulars at Antietam - a missed chance?

During the American Civil War both sides had missed opportunities to deal devistating blows to their
opponents. One such opportunity may have come to the Regulars on September 17th.

Several sources report that at about 4:30 pm Captain Dryer rode forward of the Union position. His
reconnaissance (not mentioned in his report in the Offical Records) showed that the Confederate
position to the Regular’s front was manned by only a few hundred men (the weakened brigades of
Confederate Generals Nathan Evans and Richard Garnett – numbering some 350 men). Also to the
front of the regulars were several Confederate batteries. Dryer sent a note back to Lieutenant Colonel
Buchanan, reporting the weakness of the Confederate position, and asking for permission to assault
the Confederate position with all available troops. Buchanan, in turn, sent word back to Generals
McClellan, Fitz John Porter, and Sykes – asking for permission to assault. Reportedly, McClellan and
Sykes were on the verge of giving permission when Fitz John Porter said, “Remember General, I
command the last reserve of the last Army of the Republic.” McClellan hesitated, dusk fell, and the
opportunity was lost.

The question remains – if the Regulars had been allowed to assault the Confederate positions the
afternoon of September 17th and if that assault had been successful (and supported) would the
Army of Northern Virginia have been cut in half, its flanks “rolled up” and potentially devastated,
possibly shortening the war – or would the Confederate artillery been able to provide enough support
to the Confederate line - to where it would have been sufficient to decimate any Union assault?