If the Japanese declare war just before the attack hits Pearl Harbor, it will make ZERO difference in the American response, as the declaration 15 minutes before the attack would be seen as window dressing. Had the message the Japanese embassy received been decoded & delivered on time, and had Cordell Hull taken it as a declaration of war (which it really wasn't) the results of the PH attack would have been the same as there was no way to warn them in the brief interval between when the message was SUPPOSED to be delivered. If the Japanese give real warning, ie: 24 hours, then the pacific fleet is at sea or those elements in harbor are on alert with steam up, the USAAF is flying CAP etc - therefore the whole attack plan is screwed. - Bottom line Japanese give real warning in timing of real declaration of war, no PH, if they go as planned even with real declaration its window dressing for sucker punch & no difference in American response.