Why didn't the US know Japan planned to attack Pearl Harbor?

Answer 1

Unfortunaely the American fleet was not hiding anywhere. The American fleet was very predictable with its rotations at pearl harbor. Fortunately, the American Carriers were not at Pearl Harbor when the Japanese attacked. The Japanese also a very capable human intelligence capability.

Answer 2

As an open nation, visitors were always comming to the U.S., including Hawaii. Japan really liked Hawaii and wanted to take over the islands of the Pacific. It didn't take much. They had spies and ambassadors who took pictures. It wasn't that difficult to get a good idea of where things would be.

Answer 3

President Roosevelt (FDR) provoked the attack, knew about it in advance and covered up his failure to warn the Hawaiian commanders. FDR needed the attack to sucker Hitler to declare war, since the public and Congress were overwhelmingly against entering the war in Europe. It was his backdoor to war.

FDR blinded the commanders at Pearl Harbor and set them up by -

1. denying intelligence to Hawaii (HI)
2. on Nov 27, misleading the commanders into thinking negotiations with Japan were continuing to prevent them from realizing the war was on
3. having false information sent to HI about the location of the Japanese carrier fleet.

BACKGROUND

• 1904 - The Japanese destroyed the Russian navy in a surprise attack in undeclared war.
• 1932 - In The Grand Joint Army Navy Exercises the attacker, Admiral Yarnell, attacked with 152 planes a half-hour before dawn 40 miles NE of Kahuku Point and caught the defenders of Pearl Harbor completely by surprise. It was a Sunday.
• 1938 - Admiral Ernst King led a carrier-born airstrike from the USS Saratoga successfully against Pearl Harbor in another exercise.
• 1940 - FDR ordered the fleet transferred from the West Coast to its exposed position in Hawaii and ordered the fleet remain stationed at Pearl Harbor over complaints by its commander Admiral Richardson that there was inadequate protection from air attack and no protection from torpedo attack. Richardson felt so strongly that he twice disobeyed orders to berth his fleet there and he raised the issue personally with FDR in October and he was soon after replaced. His successor, Admiral Kimmel, also brought up the same issues with FDR in June 1941.
• 7 Oct 1940 - Navy IQ analyst McCollum wrote an 8 point memo on how to force Japan into war with US. Beginning the next day FDR began to put them into effect and all 8 were eventually accomplished.
• 11 November 1940 - 21 aged British planes destroyed the Italian fleet, including 3 battleships, at their homeport in the harbor of Taranto in Southern Italy by using technically innovative shallow-draft torpedoes.
• 11 February 1941 - FDR proposed sacrificing 6 cruisers and 2 carriers at Manila to get into war. Navy Chief Stark objected: "I have previously opposed this and you have concurred as to its unwisdom. Particularly do I recall your remark in a previous conference when Mr. Hull suggested (more forces to Manila) and the question arose as to getting them out and your 100% reply, from my standpoint, was that you might not mind losing one or two cruisers, but that you did not want to take a chance on losing 5 or 6." (Charles Beard PRESIDENT ROOSEVELT AND THE COMING OF WAR 1941, p 424)
• March 1941 - FDR sold munitions and convoyed them to belligerents in Europe -- both acts of war and both violations of international law -- the Lend-Lease Act.
• 23 Jun 1941 - Advisor Harold Ickes wrote FDR a memo the day after Germany invaded the Soviet Union, "There might develop from the embargoing of oil to Japan such a situation as would make it not only possible but easy to get into this war in an effective way. And if we should thus indirectly be brought in, we would avoid the criticism that we had gone in as an ally of communistic Russia." FDR was pleased with Admiral Richmond Turner's report read July 22: "It is generally believed that shutting off the American supply of petroleum will lead promptly to the invasion of Netherland East Indies...it seems certain she would also include military action against the Philippine Islands, which would immediately involve us in a Pacific war." On July 24 FDR told the Volunteer Participation Committee, "If we had cut off the oil off, they probably would have gone down to the Dutch East Indies a year ago, and you would have had war." The next day FDR froze all Japanese assets in US cutting off their main supply of oil and forcing them into war with the US. Intelligence information was withheld from Hawaii from this point forward.
• 14 August - At the Atlantic Conference, Churchill noted the "astonishing depth of Roosevelt's intense desire for war." Churchill cabled his cabinet "(FDR) obviously was very determined that they should come in."
• 18 October - diary entry by Secretary of Interior Harold Ickes: "For a long time I have believed that our best entrance into the war would be by way of Japan."

CODES

• Purple Code - the top Japanese diplomatic machine cipher which used automatic telephone switches to separately and differently encipher each character sent. It was cracked by the Army Signal Intelligence Service (331 men).
• J-19 was the main Japanese diplomatic code book. This columnar code was cracked.
• Coral Machine Cipher or JNA-20 was a simplified version of Purple used by Naval attaches. Only one message deciphered prior to Pearl Harbor has been declassified.
• JN-25 - The Japanese Fleet's Cryptographic System, a.k.a. 5 number code (Sample). JN stands for Japanese Navy, introduced 1 June 1939. This was a very simple old-type code book system used by the American Army and Navy in 1898 and abandoned in 1917 because it was insecure. Version A has a dictionary of 5,600 numbers, words and phrases, each given as a five figure number. These were super-enciphered by addition to random numbers contained in a second code book. The dictionary was only changed once before PH on Dec 1, 1940, to a slightly larger version B but the random book was changed every 3 to 6 months- last on Aug 1. The Japanese blundered away the code when they introduced JN25-B by continuing to use, for 2 months, random books that had been previously solved by the Allies. That was the equivalent of handing over the JN-25B codebook. It was child's play for the Navy group OP-20-G (738 men whose primary responsibility was Japanese naval codes) to reconstruct the exposed dictionary. We recovered the whole thing immediately - in 1994 the NSA published that JN-25B was completely cracked in December 1940. In January 1941 the US gave Britain two JN-25B code books with keys and techniques for deciphering. The entire Pearl Harbor scheme was laid out in this code. The official US Navy statement on JN-25B is the NAVAL SECURITY GROUP HISTORY TO World War II prepared by Captain J. Holtwick in June 1971, page 398: "By 1 December 1941 we had the code solved to a readable extent." Churchill wrote "From the end of 1940 the Americans had pierced the vital Japanese ciphers, and were decoding large numbers of their military and diplomatic telegrams."(GRAND ALLIANCE p 598) Chief of Navy codebreaking Safford reported that during 1941 "The Navy COMINT team did a thorough job on the Japanese Navy with no help from the Army."(SRH-149) The first paragraph of the Congressional Report Exhibit 151 says the US was "currently" (instantly) reading JN-25B and exchanging the "translations" with the British prior to Pearl Harbor.

  In 1979 the NSA released 2,413 JN-25 orders of the 26,581 intercepted by US between Sept 1 and Dec 4, 1941. The NSA says "We know now that they contained important details concerning the existence, organization, objective, and even the whereabouts of the Pearl Harbor Strike Force." (Parker p 21) Of the over thousand radio messages sent by Tokyo to the attack fleet, only 20 are in the National Archives. All messages to the attack fleet were sent several times, at least one message was sent every odd hour of the day and each had a special serial number. Starting in early November 1941 when the attack fleet assembled and started receiving radio messages, OP-20-G stayed open 24 hours a day and the "First Team" of codebreakers worked on JN-25. In November and early December 1941, OP-20-G spent 85 percent of its effort reading Japanese Navy traffic, 12 percent on Japanese diplomatic traffic and 3 percent on German naval codes. FDR was personally briefed twice a day on JN-25 traffic by his aide, Captain John Beardell, and demanded to see the original raw messages in English. The US Government refuses to identify or declassify any pre-Dec 7, 1941 decrypts of JN-25 on the basis of national security, a half-century after the war.

• AD or Administrative Code wrongly called Admiralty Code was an old four character transposition code used for personnel matters. No important messages were sent in this weak code. Introduced Nov 1938, it was seldom used after Dec 1940.
• Magic - the security designation given to all decoded Japanese diplomatic messages. It's hard not to conclude with historians like Charles Bateson that "Magic standing alone points so irresistibly to the Pearl Harbor attack that it is inconceivable anybody could have failed to forecast the Japanese move." The NSA reached the same conclusion in 1955.
• Ultra - the security designation for military codes.

Answer 4

The United States had broken the secret code used to send messages between Tokyo and the Japanese embassy in Washington DC

Answer 5

The us didn't now the japenies were coming because they never new the japanies like them that's why

Answer 6

They didn't know. It was suspected, but not for sure. Radio communications showed something was going on, but no plans were uncovered until the attack.