Of course any General knows that even the best of plans doesn't survive the first shot. But that's why there are contingencies.
One of the greatest generals of all time said, "What distinguishes a hero is not an extraordinary man who acts courageous in the face of adversity, its an ordinary man who acts ordinary while everybody else is falling apart around him." Where did we see the sort of behavior demonstrated last weekend before? Yep that's right, from the Democratic Congressional leadership while the attacks of 9/11 were in progress.
The biggest excuse I heard about failure to evac the city last weekend was based on legal advice rendered to the mayor. Fat lot of good that's going to do him now. Moreover, the evac routes were gridlock. Why were they gridlock? That's because of lack of planning and foresight.
One of the lanes of the road wasn't designated as out-bound for emergency use only. Moreover, there should've been contingency plans afoot for vehicle wrecks to be bulldozed w/impugnity out of the evac routes. Furthermore, there was no contingency plans to open all inbound lanes (except one) to evac traffic. And finally, there was no clear decisive destination for all the people being evac'd (nor was there a logistics system in place to implement for support of these people until FedGov can mobilize).
Operation Overlord wasn't planned the week before, and they didn't move all assets into place the 24 hours prior to execution. Right now at this moment there are mountains as high as Everest of materiel, and millions of bases just wait to be used in Saudi Arabia. Why are they there? Because the Saudi's know a little bit about what I'm talking about. Somewhere near Diego Garcia are merchant marine vessels that are parked and filled with M1 Abrahms tanks. The mariners on these vessels do nothing all day long but start the engines, and maintance these tanks (and other equipment). Within 48 hrs some of that stuff can be on the ground ready to use and not just beginning to be pulled out of mothballs.