A significant constraint was that planners can only plan for eventualities that they can imagine to be realistic. While the USN had done exercises with
simulated attacks on both the Canal and Pearl Harbor, the people responsible for the defense plans of the latter didn't seem to believe that an attack by
carrier-based aircraft was a realistic threat (and they didn't seem to change their beliefs after Taranto). Similarly, their did not seem to be much
planning for attacks on coastal traffic by German submarines. Despite these failures, I think that, on the whole, US pre-war planning was reasonably good. One
area where it was excellent was in the recognition of the need to mobilize industrial resources.
Of course, everybody's outlook is influenced by the milieu of their time: disdain for democratic governments let the Japanese rulers convince themselves the US would not fight, disdain for the Japan's technical abilities convinced some of the US planners that they couldn't perform an effective attack, neglect of the German submarine threat left US coastal traffic quite vulnerable for a significant period. It's easy to look back and say "see, the planners screwed this up, and this up, because see what happened." I've certainly been in a position where I could be charged with that (see anything I've said about what seems to be the IJN's complete lack of planning for trade protection).
Of course, everybody's outlook is influenced by the milieu of their time: disdain for democratic governments let the Japanese rulers convince themselves the US would not fight, disdain for the Japan's technical abilities convinced some of the US planners that they couldn't perform an effective attack, neglect of the German submarine threat left US coastal traffic quite vulnerable for a significant period. It's easy to look back and say "see, the planners screwed this up, and this up, because see what happened." I've certainly been in a position where I could be charged with that (see anything I've said about what seems to be the IJN's complete lack of planning for trade protection).
