“Yesterday, December 7, 1941 — a date which will live in infamy — the United States of America was suddenly and deliberately attacked by naval and air forces of the Empire of Japan.”

These words opened President Franklin Delano Roosevelt’s address to the nation the day after 353 Imperial Japanese aircraft from six aircraft carriers attacked Pearl Harbor and Oahu airfields.  The attack was conducted in two waves with the second wave returning to base less than two hours after the first bomb fell.  In what, for some, must have felt like an eternity, four battleships, three destroyers, three light cruisers, and four other ships were sunk and 164 aircraft destroyed.  The attack claimed the lives of 2,403 Americans — many entombed in their own ships.  Another 1,178 uniformed service members and civilians were wounded.

The Kido Butai

The architect for the attack on Pearl Harbor was the Commander of Japan’s Combined Fleet, Admiral Isoroku Yamamoto.  Having traveled the United States extensively in the interwar years during his studies at Harvard University and two tours as Naval Attache in Washington D.C., Yamamoto was no stranger to American culture.  He had observed, first-hand, the industrial capacity and production potential of the United States and was strongly opposed to war with the powerful nation.  Nevertheless, as diplomatic relations with the U.S. began to break down, war seemed inevitable.

Admiral Isoroku Yamamoto, Commander, Combined Fleet

Admiral Yamamoto was a strategic thinker.  He was convinced that if war was to break out, Japan must strike first and strike hard if there were to be any hope of victory.  A war of attrition would most certainly favor the American fleet.  In the fall of 1941, he told Prince Fumimaro Konoye, “I shall run wild considerably for the first six months or a year, but I have utterly no confidence for the second or third years.”  His concept of what a first-strike might look like was met with opposition.

Japan’s war plans, similar to those of the United States, all centered on a climactic surface engagement off the home waters of Japan.  The prevailing theory within the Imperial Japanese Navy was consistent with Mahanian theory:  that any war at sea would culminate in a clash of two opposing concentrations of sea power in a single, decisive battle.  As such, they placed all of their hopes in a naval force dominated by battleships.  Not everyone in the Admiralty agreed.  There were some, Rear Admiral Inoue Shigeyoshi chief amongst them, who argued for the complete removal of both battleships and aircraft carriers in favor of long-range, land-based bombers.

Yamamoto was somewhere in the middle.  Though not willing to completely dismiss the value of battleships — he, nevertheless, viewed them as “paper tigers” which were quickly being made obsolete by the aircraft carrier.  He believed that the key to success in any battle would be naval aviation and not the mighty battleship.  Future wars would be won by a combination of land-based and carrier-based aircraft.  It was based on this conviction that he conceived the organization of the First Air Fleet which would form the basis for the Kido Butai or, striking force.

Comprised of the carriers Akagi, Kaga, Hiryu, Soryu, Zuikaku, and Shokaku along with 26 battleships, cruisers, destroyers, submarines, and tankers — the Kido Butai was the most formidable force to have ever sailed the Pacific Ocean — or any other ocean for that matter.  Embarked in her six aircraft carriers were 464 fighters, torpedo planes, and dive bombers.  On November 25, 1941, the Kido Butai departed Japan’s northern Kurile islands enroute Pearl Harbor.

Admiral Chuichi Nagumo

Commanding the Kido Butai was Admiral Chuichi Nagumo.  Finding Nagumo would prove to be the most challenging and disappointing pursuit Station HYPO and its sister stations would ever attempt.

Finding Nagumo

Rochefort and his team were watching all of this unfold from within the confines of the dungeon.  However, the picture that they were seeing was not nearly as clear as the one painted above.  They were only able to infer a fraction of what was occurring.  Though traffic analysis had indicated an unprecedented reorganization, the exact composition and operational employment of such was uncertain.  The impact of such uncertainty would be far-reaching.

Prior to the formation of the Kido Butai, Japan’s ten aircraft carriers were operationally assigned to one of five carrier divisions numbered sequentially from 1 to 5.  Each of these five carrier divisions had specific destroyer divisions assigned.  When a carrier division deployed, the same destroyer division would always accompany them.  This was significant for a traffic analyst’s job.  Elliot Carlson writes:  “Specific destroyers were assigned to specific carriers and battleships.  Because there were more destroyers than carriers and battleships, it was often easier to obtain the callsigns of those vessels.  So when traffic analysis showed a certain destroyer moving — one associated with a particular carrier — analysts inferred that a carrier division was also moving.”  Yamamoto’s shakeup of the Combined Fleet organization rendered the above method of analysis useless.  Everything that HYPO’s analysts knew about how the IJN was organized had now changed.

Kido Butai attack formation

On November 1, 1941, the IJN implemented a callsign change for all naval units.  This move, though not unexpected, further stymied HYPO’s traffic analysts.  Two days later, on November 3rd, the first use of the callsign “First Air Fleet” was heard in message traffic.  Rochefort had no knowledge of any such organization within the IJN and could only guess as to its meaning or significance.  He continued to feed all of this information to Admiral Kimmel through his intelligence officer, Edwin Layton, but was growing increasingly frustrated at his inability to make any of it operationally relevant.  All indications pointed to something significant occurring within the IJN, but there simply was not enough information.

Such frustrations would only get worse over the coming weeks.  The 3 November intercept of the First Air Fleet would be the first and last time they were ever detected in radio traffic.  The Kido Butai was operating under complete radio silence.  Additionally, the IJN had implemented more strict communication security practices for all traffic originating in Tokyo and intended for fleet units.  The messages would no longer address specific units and the callsigns would be embedded in the encrypted message itself.  Though they tried, Rochefort and his team would not find Nagumo and his striking force until it was too late.

Failure of Imagination

Much has been investigated, researched, and written about who was culpable for the failure of intelligence to warn of the impending Japanese attack on Pearl Harbor.  The theories are many and varied.  The determination of which theory holds the most water is not the intent of this writing.  Though not all are addressed in this text, there were many contributing factors to what has been considered a gross intelligence failure.  Author, Elliot Carlson, devotes an entire chapter to this subject which is very appropriately titled:  “Comedy of Errors.”


Some of the factors contributing to this comedy of errors included:  Army-Navy relations and sharing (or not sharing) of information, division of effort for the various Japanese codes amongst the three key stations (HYPO, CAST, and NEGAT), Japanese communications security and deception, inability to decrypt JN-25(b) — the IJN’s main operational code (this factor is debatable considering the radio silence posture under which the Kido Butai was operating), Rochefort’s disinterest in breaking the diplomatic code, and the list goes on.

Perhaps a better explanation is one inferred by historian, John Lundstrom, who opined:  “What was going on here was a failure of imagination… It was unthinkable because that wasn’t the way the [U.S.] Navy would do it.”  In the late months of 1941, no one, including Rochefort, imagined the Japanese would make such a bold and daring move.  Rochefort believed Yamamoto would attack U.S. interests or allied territories elsewhere in the Pacific — never did he imagine Japan would strike at the heart of the U.S. Pacific Fleet in Hawai’i.  He was wrong.

Commander Joe Rochefort’s job was to track the Imperial Japanese Navy and to provide operationally relevant intelligence to CINC PACFLT.  In this, he failed.  Fueled by guilt and resolve, he would not fail again.

*Stay tuned next week for the third installment of the “Path to Midway” series:  “Tracking the Imperial Japanese Navy.”


*The title, “A Failure of Imagination,” is taken, in part, from a quote by Pacific War Historian John Lundstrom regarding the U.S. Navy’s failure to connect the dots from the various, albeit ambiguous, indications that Japan was going to attack Hawai’i.

**The above writing is an extremely condensed version of the story leading up to Japan’s attack on Pearl Harbor.  For further reading, I highly suggest any one, or all, of the references below.  Elliot Carlson’s book focuses more closely on the specific efforts of Station HYPO.

[1]  Carlson, Elliot.  Joe Rochefort’s War:  The Odyssey of the Codebreaker Who Outwitted Yamamoto at Midway.  Annapolis, MD:  Naval Institute Press, 2011.  Print.  [pp. 91-113]

[2] Symonds, Craig.  The Battle of Midway.  New York, NY:  Oxford University Press, 2011.  Print.

[3] Toll, Ian.  Pacific Crucible:  War at Sea in the Pacific, 1941-1942.  New York, NY:  W. W. Norton & Company, 2012.  Print.