When it comes to war you plan for all possible contingencies, if you don't or cannot, then you have no business being in the war business.
How long do you take to do this planning for all possible contingencies? You can never finish planning if you have to consider all possible contingencies. How much planning should we have done for the possibility that an asteroid might impact Basra before its capture, during the battle for Basra or after its capture?
You only plan for likely events. How do you determine what events are likely? You make assessments based on the intelligence available. Unfortunately, we had faulty intelligence. It has happened countless times throughout history that military decision were made on faulty intelligence, and it will continue to happen as long as men fight each other.