... move through Crampton's gap, and their duty was first to cut off, destroy, or capture McLaws' command, and relieve Colonel Miles at Harper's Ferry; if too late to aid Miles, they were to turn toward Sharpsburg to prevent the retreat of Longstreet and D. H. Hill, who were to be attacked by the main body. All the rest of McClellan's army set out, by way of Turner's gap and Fox's gap, for Boonsboro. This ...