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According to "British Intelligence in the Second World War: Security and Counter-Intelligence", by Francis Harry Hinsley and C.A.G. Simkins, Tricycle, a British double agent, complained that the FBI ignored the "obvious warning" of the forthcoming attack on Pearl Harbor (read more below ↓).

Also, according to "Conspiracies and Secret Societies: The Complete Dossier", by Brad Steiger & Sherry Steiger, it seems that the Japanese diplomatic code had been broken by FBI and almost all messages between Tokyo and its embassy in Washington were being intercepted and understood by Washington (read more below ↓).

Therefore, as it is possible to argue, there is no longer any doubt that FBI and, perhaps, Franklin Delano Roosevelt, were aware that an attack on Pearl Harbor was developing and that it was scheduled for December 7th.

Then a terrible question arises: was the attack on the American forces at Pearl Harbor totally unexpected?

If not, why did no one alert defense forces to protect Pearl Harbor?

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This is a controversial question that people have written entire books about (some of them are quite good reads, actually). Although you can get a good brief answer here, if you're really into the topic, you should get a good book about it. – American Luke Aug 10 '13 at 17:02
and you should not believe the conspiracy theorists... Yes, in 20-20 hindsight there were indications that the Japanese were planning a move against the US fleet. At the time however those were not recognised as such. – jwenting Aug 10 '13 at 17:30

Although the Japanese attack was unexpected in its timing, The US Navy was well aware:
(a) that the Japanese were in the habit of attacking before a formal declaration of war; and
(b) that a surprise attack on Pearl Harbor was both possible and likely to be devastating, having itself simulated such an attack several times over the past 15 years as outlined here

Fleet Problem XIV

“The 1933 problem was designed to simulate a war in the Pacific, one initiated by carrier operations. Anticipating that Japan would attack before formally declaring war (as she had done against Russia in 1904), the scenario envisioned the sortie of the Japanese fleet eastward across the Pacific. This fleet, its sinister designation Black, had ominously prescient orders: “To inflict maximum damage on the PEARL HARBOR NAVAL BASE in order to destroy or reduce its effectiveness.”

All in all, I find it no coincidence that the Navy carriers spent as much time as possible at sea instead of Pearl, especially as tensions with Japan increased.

The claim is made below that the US Navy was unappreciative of aircraft carriers prior to Dec. 7, 1941. I submit this as evidence to the contrary (from Wkipedia, my emphasis).

In 1934, the then Chief of Bureau of Aeronautics, Navy Admiral Ernest King offered Halsey command of the aircraft carrier USS Saratoga, subject to completion of the course of an air observer. Captain Halsey elected to enroll as a cadet for the full twelve-week Naval Aviator course rather than the simpler Naval Aviation Observer program. "I thought it better to be able to fly the aircraft itself than to just sit back and be at the mercy of the pilot." Halsey earned his Naval Aviator's Wings on May 15, 1935 at the advanced age of 52, the oldest person to do so in the history of the U.S. Navy.

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Carriers at that time were not considered very important ships by the USA navy. Japan felt differently (and were about to teach the Americans that lesson), but the USA essentially just had them because their opponents had them. So I find it unlikely in the extreme that their absence that day was a result of any clever plan on the USA's part. Hanlon's Razor should be applied here. – T.E.D. Aug 10 '13 at 19:05
@T.E.D.: Complete horse hockey! That may have been true of SecNav, but the up-and-comers like Halsey and Towers had known better for years. – Pieter Geerkens Aug 10 '13 at 19:57
@T.E.D. The relevant thing is not if they "appreciated" carriers, but if they thought Japans fleet had the range to attach Pearl Harbor or not. – Lennart Regebro Aug 11 '13 at 4:01
the USN was heavily divided. One school was all about battleships, and another school about airpower. And the latter itself was divided between carrier power and land based air. And then there were the submarine advocates, and a small group wanting to go all coastal with no more blue water navy in the light of US isiolationism making that an unneeded expense and provocation towards other countries. – jwenting Aug 11 '13 at 5:52
@jwenting Can you add some details about this last group? – Felix Goldberg Aug 12 '13 at 9:21

Indeed, Japanese diplomatic codes had been broken. But the message sent to the Japanese Embassy in Washington, intended to be delivered before the attack (but in fact delivered later) did not contain a formal declaration of war, so although Washington knew a few hours before the attack that diplomacy was coming to an end, and war was coming, they did not receive a declaration of war, and did not expect an attack. (1)

It is sometimes claimed that the US leadership should have known that Japan would attack before a declaration of war was made, but from the US standpoint Japan and the US was still in active negotiations. The last part of the message did end those negotiations, but that part had not yet been decoded when the attack happened.

In addition to that, the Japanese fleet had been travelling under complete radio silence, and had not been detected by the united States, so the US did not know that there was a fleet within attacking distance. (2)

So yes, the attack was completely unexpected. The US thought Pearl Harbor was safe, well out of the reach of the Japanese, and negotiations was still underway.

(1) John Toland, Infamy: Pearl Harbor and Its Aftermath

(2) Donald M. Goldstein, Katherine V. Dillon: The Pearl Harbor Papers

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If you follow that link and actually read the message, absolutely right that this was not a declaration of war. All it says is "it is impossible to reach an agreement through further negotiations." – T.E.D. Aug 19 '13 at 23:03

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