Success in war is the result of recognizing, and taking advantage of, the opportunities that present themselves which cannot have been forseen. The collapse of Iraqi resistance and the disintegration of the Iraqi military was one of those opportunities.
To argue that because it wasn't specifically enumerated on some sort of to do list or job description (your words), taking advantage of the opportunity to destroy Iraqi military capablity completely, reflects an utter lack of strategic sense. It is Emerson's foolish consistency that is the hobgoblin of little minds, much beloved of small statesmen, philosophers and divines.