Although an immediate Union attack on the morning of September 16 would have had an overwhelming advantage in numbers, McClellan's trademark caution and his belief that Lee had as many as 100,000 men at Sharpsburg caused him to delay his attack for a day.
Selv om et øjeblikkeligt angreb om morgenen den 16. september ville have haft en overvældende fordel i antal var McClellans særkende hans forsigtighed, og hans tro på at Lee havde over 100.000 mand fik ham til at udskyde sit angreb en dag.