You are straying into Truther Territory, there partner. There was no clear, unambiguous signal saying “attack Pearl Harbor”. Navy intelligence had lost track of the ships in the attack force. Their Morse Code operators had been taken off and placed on islands in the South China Sea, and given dummy traffic to send, since US Huff Duff operators would recognize a particular operator by his “fist”. In retrospect, their are indicators that point to an attack, but hindsight is 20/20. If FDR had known about the attack, he could have done a better job of absorbing it, and would have been in a position to inflict great damage on the Japanese early in the War.
I always felt that the Pearl Harbor attack, as serious as it was, was not daring enough. On December 8, 1941 there was more oil in Hawaii than in Japan. Had the Japanese attacked the oil storage facilities, it is likely that they would never have lost Guadalcanal, and some of the subsequent battles, and the war in the Pacific, would have been lengthened by at least a year. Or more. What if the Japanese had invaded the unprotected beaches of Hawaii a few days after Pearl Harbor, supported by aircraft carriers and naval gunfire, possibly taking the Pearl Harbor out of U.S. hands, and possibly even seizing significant oil reserves?
Hypothetically, I've always thought the best course of action for the Japanese would have been, rather than attacking HI, to simply by pass the Philippines on their way to SE asian oil. Although that would have violated a cardinal rule of war making (leaving an enemy in their flank) not only was there was no great enthusiasm among Americans for getting involved in another war, we weren't big fans of the colonialists either. They might have gotten away with it without giving Roosevelt a political opportunity to involve us.
“... There was no clear, unambiguous signal saying attack Pearl Harbor. ... indicators that point to an attack, but hindsight is 20/20. If FDR had known about the attack, he could have done a better job of absorbing it, and would have been in a position to inflict great damage on the Japanese early in the War.
... What if the Japanese had invaded the unprotected beaches of Hawaii a few days after Pearl Harbor, supported by aircraft carriers and naval gunfire, possibly taking the Pearl Harbor out of U.S. hands, and possibly even seizing significant oil reserves?”
Cryptanalysis is more difficult than many casual observers can appreciate. The Allied (in reality, Polish/British/US) successes in this field came later in WWII: in 1941, none of the infrastructure nor procedures existed. Anyone who thinks - even for so long as a millisecond - that it was even possible for the US government to discover what Imperial Japan was up to, is committing the most grievous errors imaginable.
Gordon Prange (and his inimitable co-authors, whose names refuse to return to my memory at the moment) has shown conclusively that no actions the US could have taken would have influenced Imperial Japan one way or another: the decision to go to war was taken by the Japanese leadership.
Recent analyses and informed conjecture have conceded that no amount of tactical warning could have saved US forces from the mauling they endured on 7 December 1941, and the days immediately following. There simply were no command and control systems to receive alerts, assess attack strength/objectives, assign defense assets, or direct them in battle.
All speculation about “what might have happened” is of course uncertain, but it’s quite possible that had the USN Pacific Fleet sortied from Pearl Harbor in time to meet the oncoming Imperial Japanese Naval attack fleet in a full-scale open-ocean battle, the results would have been far worse for the Americans. And every vessel lost in that clash could never have been salvaged, sunk as they would have been in deep water.
What truly cooked the American goose that day was lack of air-mindedness on the part of local commanders: the US Navy was preparing to meet a naval attack, and the US Army was preparing to meet a ground attack (an invasion force, as Lonesome in MA suggested). Never the twain did meet.
And - most ruinously - blinkered thinking could not be blamed on personal flaws of local commanders. Both Walter C. Short and Husband E. Kimmel were products of their separate service traditions and corporate culture. Their approach to waging war, while admirably traditional, was outdated decades before. The late Billy Mitchell predicted so in hideous detail in the 1920s, and moreover had the bad manners to prove it.
And no one can guarantee that blunders on such a scale can never descend on us in the future, as we persist in clinging to brainlessness ... the family of Admiral Kimmel persevered in petitioning the Navy Dept to get his name cleared, and USN leadership agreed at last. Family members of William Mitchell have made several attempts to get his court-martial conviction reversed, but in inexplicable contrast, DoD has always turned them away.
From "And I Was There: Pearl Harbor and Midway Breaking the Secrets" by Layton (with Pineau and Costello), circa 1985.
Page 317: "... 'How do you know it's Akagi?' I asked. Joe explained, 'It's the same ham-fisted radio operator who uses his transmitting key as if he is kicking it with his foot'"
[See additional "fist" comments, for example, on page 174 and pages 229-230.]
As the story about IJN "radio deception" has as a major pillar that the Japanese naval radio operators from the Striking Force remained in Home Waters; the above shows that at least one remained on Akagi for the attack, transmitted, and was recognized. So much for that myth.
Also see page 547, note 19, for a bit more from a chapter of CSP-1494-A "Did the Japanese Paint Us a Picture" (The OP-20-G discrediting campaign against Rochefort and his men?)
But then, there is the tale of placing some of the Kido Butai's transmitters on destroyers which then sailed around Home Waters, ... and so on.
Of course, your mileage may vary.