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A Certain Grandeur:  U.S. Communications Intelligence In The Pacific War, 1941-1945
CSC 1992
Title:  A Certain Grandeur:  U.S. Communications Intelligence
in the Pacific War, 1941-1945
Author: LCDR P. R. Gardella, United States Navy
Thesis: Naval communications intelligence was quite
successful during World War Il, despite the lack of high
technology tools.  It is therefore instructive to look back in
time at the role communications intelligence played in
operations and decision-making in the Pacific theater during
that conflict.
Summary:  There were six primary areas in which COMINT played
a significant role in the U.S. effort: the buildup to war,
monitoring of enemy activity in the Pacific, U.S. offensive
operations, the submarine war, battle damage assessment, and
forward-deployed tactical support to naval forces.
    While the Japanese diplomatic code had been broken by late
1941, other codes that would have been more useful to the
Pacific Fleet commander were deemed of lesser priority by the
communications establishment in Washington, and so were either
not yet broken or were not being decoded and passed on to Pearl
Harbor.  Had they been, the Pacific Fleet might have been more
prepared for the Japanese attack.
    Through decryption of messages and traffic analysis, naval
intelligence officers at Pearl Harbor determined that the
Japanese were attempting to expand southward in the Pacific
Ocean and, later, to attack Midway.  A U.S. decision to block
these moves resulted in battles at Coral Sea (May 1942), Midway
(June 1942), and Guadalcanal (August 1942).
    Monitoring of Japanese radio nets serving the enemy-held
Pacific islands gave the U.S. much intelligence of enemy
strength and the nature of operations.  U.S. naval commanders
were able to use this information to launch air strikes on
several Japanese bases.
    COMINT on Japanese merchant ship locations and movements was
passed to patrolling U.S. submarines, who waged a deadly war on
Japanese commerce, severely restricting the import of critical
strategic resources to the Japanese mainland.
    Interception of Japanese damage and loss reports allowed
U.S. commanders to avoid being deceived by exaggerated reports
of American victories, ensuring that U.S. commanders had an
accurate picture of remaining enemy strength.
    Mobile COMINT detachments were assigned to flagships to
provide COMINT of a more tactical nature to the embarked
commanders, allowing them to be forewarned of enemy air attacks
on their forces.
                      A CERTAIN GRANDEUR:
Thesis Statement:
	Naval communications intelligence was quite successful
during World War II, despite the lack of high technology tools.
It is therefore instructive to look back in time at the role
communications intelligence played in operations and
decision-making in the Pacific theater during that conflict.
I.	Areas of Contribution
	A.	Prelude to War
	1.	Codebreaking Efforts
		a.	PURPLE Code
		b.	J-19/PA-K2 Codes
		c.	JN-25 Code
        B.  Monitoring Enemy Activity
            1.  Indochina and South China Sea
            2.  New Guinea
            3.  Midway
            4.  Solomon Islands
        C.  Support to U.S. Offensive Operations
            1.  Gilbert Islands Raid
            2.  Marshall Islands Raid
            3.  Palau Islands Assault
D.  Support to U.S. Submarine Operations
    1.  Attempted Attack on Naval Shipping
    2.  Attacks on Merchant Shipping
    3.  Indirect Benefits
E.  Support to Battle Damage Assessment
    1.  Japanese After-Action Reports
    2.  "Victory Diseased"
F.  Tactical Support to U.S. Naval Operations
    1.  Marshall Islands Raid
    2.  Doolittle Raid
    3.  Battle of Philippine Sea
    4.  Okinawa Campaign
                              A CERTAIN GRANDEUR:
	Naval intelligence officers of the past two or three decades
have grown up professionally in a world of satellites and
computers, comfortable with and, unfortunately, reliant upon
this high technology for communications intelligence (COMINT)
collection and analysis.  This often causes intelligence
officers to regard the efforts of our forerunners during World
War II as being archaic and therefore unworthy of study.  Yet,
naval communications intelligence was quite successful during
World War II, despite the lack of high technology tools.  It is
therefore instructive to look back in time at the role
communications intelligence played in operations and
decision-making in the Pacific theater during that conflict.
    There were six primary areas in which communications
intelligence (or, as it was called at the time, Radio
Intelligence) played a significant role in the U.S. effort: the
buildup to war, monitoring of enemy activity in the Pacific,
U.S. offensive operations (the Island-hopping campaign), the
submarine war, battle damage assessment, and forward-deployed
tactical support to naval forces.
    By the time of the Pearl Harbor attack, the U.S. had been
monitoring Japanese communications for many years, with major
cryptanalytic (codebreaking) efforts being conducted in three
places: Corregidor, in the Philippines (codenamed Station Cast,
which later moved to Australia and became known as Station
Belconnen), Pearl Harbor (Station Hypo), and Washington, D~
(Station Negat).  A British effort was also underway at
Singapore, which later moved to Ceylon.  The Japanese used
several codes, each assigned to a different type of traffic.
For example, diplomatic traffic was encrypted by a device called
the Type 97 machine, and this code was given the cryptonym
PURPLE by the Americans.  The intercepted messages and
intelligence derived therefrom were collectively known as
"Magic," a subset of "Ultra," which was any intelligence derived
from decryption.  The code was broken about a year before the
Pearl Harbor attack, so American political leaders were privy to
much of the diplomatic reports and instructions passed between
the Japanese ambassador in Washington and Japanese leaders in
Tokyo.  This put U.S. leaders in an enviable position:
    With the PURPLE machine, it was sometimes possible
    to deliver decrypted Japanese dispatches to
    Secretary-of State, Cordell Hull, and the President
    of the United States before Japanese code clerks
    could deliver them to their own ambassadors. (2:45)
Unfortunately, though this breakthrough gave U.S. State
Department negotiators a terrific advantage in the political and
economic discussions ongoing with Japan, it was not helpful in
providing clear warning of the impending surprise attack.  The
diplomatic advantage provided by PURPLE, however, was so
compelling that other Japanese codes were largely ignored.  Two
such codes were designated J-19 and its replacement PA-K2, and
were used to encrypt traffic between the Japanese consulate in
Honolulu and government leaders in Tokyo.  These codes had been
broken but were accorded such a low priority that incoming
messages were often not decrypted and translated for many days,
and little significance was attached to their contents.  This
was a disastrous mistake, because a series of messages sent from
Tokyo to the consulate in Honolulu during the last four months
of 1941 (the final message being sent on December 3) ordered
very detailed surveillance and reporting of ship movements in
Pearl Harbor, clearly indicating something was afoot.  Had
Pearl's Station Hypo seen these messages, history might have
been much different, but they did not.  The Navy's Radio
Intelligence effort was being directed from Navy headquarters in
Washington, OP-20-G (Station Negat's official, unclassified
name), and they had directed that Station Hypo concentrate on
breaking the Japanese flag officer's code, which proved to be
unproductive.  In the meantime, Hypo relied upon OP-20-G to
inform them-of any significant traffic in the J-19 or PA-K2
codes, but OP-20-G neglected to pass on any of the messages in
this crucial late-1941 series.  The reason given by OP-20-G
leadership was that they were not aware Pearl Harbor was not
receiving this traffic.  Whether or not this is true is a matter
of speculation, but it exacerbated the already severe friction
existing between OP-20-G and Station Hypo, which continued
unabated throughout the war.
    The most valuable Japanese code was the naval operations
code, designated JN-25.  OP-20-G and the British in Singapore
had been trying to break this code since its introduction in
June 1939.  Station Cast, in the Philippines, also began working
JN-25 in April 1941.  None of these organizations realized more
than limited success prior to the outbreak of the war, however.
OP-20-G finally permitted Station Hypo to abandon its efforts
against the Japanese flag officer's code and to start working on
the operational code ten days after the attack on Pearl Harbor.
Significant success was finally realized by Station Hypo In
mid-January 1942, and they became, from that point on, the real
center of the cryptanalytic effort against Japan.
    Though failing to prevent the surprise attack that started
the war, COMINT came to play a crucial and continuing role in
U.S. operations.  Even before the war started, Navy
cryptanalysts had broken the Japanese weather code, which
provided more valuable information than lust weather.  As one
intelligence officer observed, "...Japanese activity had a habit
of following the trail blazed by the establishment of weather
stations." (2:17)
    Without the ability to read the JN-25 naval code, little
specific information concerning Japanese operations and intent
was being directly plucked from intercepted messages prior to
the start of the war.  However, through "traffic analysis" a
fairly accurate picture was being assembled.  Traffic analysis
is the technique of analyzing the unencrypted parts of a
message, such as the originator and addressees, as well as the
volume of messages sent, to get a general "feel" for what the
enemy's next move might be.  This analysis at Station Hypo
suggested there were Japanese fleet concentrations at Formosa
and Hainan Island in the South China Sea; in the Palau Islands,
off the southeastern end of the Philippines; and in the Marshall
Islands, in the central Pacific.  This latter concentration was
estimated to contain many submarines as well as at least one
aircraft carrier.  Combining this information with knowledge of
the status of diplomatic negotiations between Tokyo and
Washington led Pacific Fleet intelligence officers to estimate
that the Japanese would soon conduct a simultaneous attack on
Indochina and the Netherlands East Indies while deploying a
submarine and aircraft carrier defensive screen in the
mid-Pacific to block any American interference.  This analysis
turned out to be essentially correct, but while U.S.
intelligence was preoccupied with the Japanese thrust to the
south, the Pearl Harbor attack force sortied from Japanese
waters under radio silence.  While it was noted at Station Hypo
that a major part of Japan's carrier force was conspicuously
absent from the message traffic, no one associated this with an
impending attack on Pearl Harbor.
    In the opening months of 1942, while the Pacific Fleet was
reconstituting and no major U.S. offensives were in the offing,
naval intelligence continued monitoring Japanese military
expansion in the South Pacific.   The Japanese naval operational
code, JN-25, was being partially read by this time, as were
several lesser codes, and references to several of the islands
in the New Guinea area began cropping up.  A weather station was
established on Woodlark Island, just east of New Guinea, and
several geographic points on and around eastern New Guinea
turned up in traffic.  Most importantly, a two-letter designator
for an undetermined place, MO, was seen with increasing
frequency.  Through analysis by officers at the Combat
Intelligence Unit (Station Hypo's official, unclassified name)
at Pearl Harbor, MO was evaluated to be Port Moresby, a major
port on the southeast coast of New Guinea.  It was beginning to
look as though the Japanese eastward expansion had halted for
the time being and they were reorienting their offensive effort
to the south.  Capture of Port Moresby would put the Japanese
one step closer to Australia, so a decision was made to counter
this move.  COMINT indicated the main Japanese carrier striking
force, which had been operating against the British in the
Indian Ocean, was returning to the Central Pacific and was due
to arrive at Truk on April 28, 1942, so the Combat Intelligence
Unit concluded the New Guinea campaign would commence in early
May, and so advised Admiral Nimitz, who rushed two carrier task
forces into the Coral Sea to block the Japanese.  On May 4,
1942, the Battle of the Coral Sea began, after COMINT brought
the two fleets together.
    Even before the first shots were fired at Coral Sea, COMINT
indications of something afoot in the mid-Pacific were
received.  First, decrypted traffic revealed Tokyo's interest in
maps of the Aleutians.  Then, references to another campaign
were seen with increasing frequency.  Intercepted messages
detailing movements of ships in the mid-Pacific began to
indicate an impending operation in that area, unconnected to the
New Guinea operation.  The appearance of the phrase "invasion
force" followed by the geographic designator "AF" in a partially
decrypted message on May 14, 1942 finally convinced Nimitz that
a mid-Pacific operation was indeed being planned.  Through a
complicated analysis of other decrypted messages, the Combat
Intelligence Unit determined AF to be Midway Island.  Similar
analysis eventually suggested the attack would occur in early
June.  Then, on May 20, 1942, the event occurred that has made
famous the breaking of the Japanese code: the operation order
for the Midway attack was intercepted and decrypted, setting the
stage for the battle that was the turning point of the war in
the Pacific.  Ironically, just a week before the battle, the
JN-25 code (B version) was changed to the JN-25C version, which
precluded any further decrypts until well after the conclusion
of the battle.  But it mattered little, since no significant
changes to the Japanese plan were made after the code change.
    The Battle of Midway was, of course, a brilliant
intelligence coup for the United States, but the late May 1942
code change was to cause problems after all.  Few, if any, JN-25
messages were being decrypted throughout that summer, but
traffic analysis indicated the Japanese were beginning to move
into the Solomon Islands.  Early in July 1942, a force of  
fifteen Japanese warships and a dozen transports (a sure sign of
a ground offensive) were discovered, through traffic analysis
and radio direction finding, to be in the area of Rabaul and
Tulagi, in the Solomons.  Shortly thereafter, a Special Landing
Force (the Japanese equivalent of the Marine Corps) was ordered
to proceed to Rabaul, where still more assault-related combat
shipping was noted.  Convoy, submarine, and air activity was
later detected in the New Britain/Rabaul/Tulagi area.
Confirming all these indicators was a document captured in New
Guinea on July 11 that showed two Billeting Detachments were
going to Guadalcanal, suggesting an occupation was imminent.
Once again, U.S. leadership was determined to block further
expansion southward by the Japanese, so plans for a U.S.
invasion of Guadalcanal were hurriedly developed.  Just as with
Midway, and Coral Sea before that, COMINT had been the backbone
of U.S. operational planning.
    The Japanese JN-25C naval operational code was changed to
JN-25D on August 14, 1942, after being in use for only two and a
half months, most likely due to a newspaper article in the U.S.
which revealed how the breaking of the Japanese code had
contributed to the victory at Midway.  This meant that COMINT
support during the battle for Guadalcanal was restricted to
traffic analysis.  The new version of the code was not
penetrated until the fight for Guadalcanal was nearly over in
the opening months of 1943.  However, cryptanalysis supported
every significant U.S. operation from that point on.
    For example, American commanders received feedback on the
results of carrier raids on Makin and Tarawa atolls, in the
Gilberts, in September 1943.  By intercepting and deciphering
Japanese after-action reports, U.S. intelligence learned that
both installations had suffered heavy personnel casualties,
eight out of sixteen of the aircraft at Tarawa's Betio Island
airfield were destroyed, and a maru (merchant ship) and three
picket boats were sunk In the Tarawa lagoon.  Thus, a fairly
clear picture of the strength of the islands' defenses was
available for planners.  Additionally, prior to the assault on
these atolls in November 1943, much was discovered through
cryptanalysis about the number and condition of the defenders.
One intelligence officer noted,
    The garrisons in the Marshalls and Gilberts
    regularly reported their strength in messages
    breaking down the figures into combat effective,
    sick, and wounded.  Available stores, including
    ammunition, were also reported on a monthly or
    weekly basis, depending on the situation.  The
    islands that had airfields reported on the number
    of planes that were fully operational, partly
    operational, and out of order, as well as the
    up-to-date fuel supply.  From all these messages
    directed to higher headquarters we could obtain an
    overall view of the situation in the principal
    islands. (6:165)
    Prior to a carrier raid on Kwajalein and Wotje, in the
Marshalls, in December 1943, cryptanalysts at Fleet Radio Unit,
Pacific (FRUPac, as the Radio Intelligence section of the Combat
Intelligence Unit in Pearl Harbor was called by this time) were
monitoring Kwajalein radio circuits and discovered a critical
vulnerability in the way air searches were carried out by the
Japanese air force on that island:
    Since the Japanese could not maintain a full-
    perimeter search with the planes they had
    available, they left the northern sectors vacant
    and concentrated their searches to the south and
    southeast, from which directions they expected an
    attack. (2:157)
This information was relayed to Admiral Pownall, commanding the
carrier strike force, who brought his ships into launch position
from the northeast, thereby achieving complete surprise.  The
attack sank four ships and destroyed fifty-five Japanese
aircraft.  The Kwajalein radio circuits had still more treasure
to give up, however.  The Japanese air squadrons in the
Marshalls reported their air losses and air strength to Tokyo
daily.  By intercepting and decrypting these messages, FRUPac
was able to maintain an accurate picture of the air situation in
that area of the Pacific.
    What may be considered Ultra at its finest, however, was a
message intercepted on July 28, 1944 from the Japanese 14th
Division to Tokyo:
    Entitled "Disposition of Forces," it supplied all
    the details, unit by unit, of the garrisons on the
    main island of Palau and the four subsidiaries,
    Peleliu, Yap, Kotor and Angaur.  Even the location
    of the mobile reserve was mentioned....The [U.S.]
    attack began. . on September 15, with landings by
    the 1st Marine Division on Peleliu.  When all was
    over, it turned out that with the exception of a
    single company no unit was encountered in the
    Palaus which was not identified in the Japanese
    message. (4:257)
Unfortunately, intelligence, no matter how complete, was often
not enough to make for an "easy" victory, for in the Palaus
1,792 Americans died and another 8,011 were wounded before the
island group was secured.  One can only guess at the grisly toll
in human life that might have resulted had the landing been made
with little or no knowledge of the opposing forces.
	The first application of COMINT to the submarine war came in
February 1942, when FRUPac, having broken a minor code used by
the Japanese port director at Truk naval base for reporting ship
arrivals and departures, discovered a message revealing the
departure of an aircraft carrier.  Through very informal
coordination with the staff of the Pacific Submarine Force
(SUBPAC), the information was sanitized and transmitted to USS
GRAYLING, a submarine patrolling off Truk, to enable her to
intercept and attack the carrier.  The submarine later sighted
the carrier but was unable to obtain a favorable firing
position, so no attack took place, but a valuable precedent had
been set, paving the way for close cooperation between
intelligence and the submarine force that lasted throughout the
    This Ultra information was perhaps more valuable in the
submarine war against the Japanese merchant marine than against
combatants.  Lacking many crucial raw materials, Japan depended
on her merchant fleet to deliver these resources from the East
Indies and Southeast Asia, as well as to supply her ever-
broadening empire stretching across the Pacific.  Through the
use of a submarine blockade, the U.S. hoped to strangle Japan by
cutting off this flow of materials and supplies.  Merchant fleet
communications were encrypted using a separate code, dubbed the
"maru code" by the Americans (since "Maru" was part of the name
of all Japanese merchant ships).  This code was broken in early
1943, which provided naval intelligence officers with knowledge
of Japanese merchant ship locations, routes, schedules, and
sometimes ship names and cargo.  This information was passed to
U.S. submarines with devastating effects:
    ... during ten months of 1943 codebreakers were able
    to alert submarine commanders to over eight hundred
    targets....about 350 of these ships were actually
    sighted and of these. .33 were sunk and 56 damaged.
    Two hundred thirty-four ships were left unmolested,
    usually because the attacking submarine was
    engaging other targets or was in an unfavorable
    position. (5:129)
Though no records exist which indicate precisely what percentage
of total shipping sunk by submarines over the course of the war
was attributable to COMINT, one estimate places that number at
fifty percent. (5:135)  Based on the frequency with which U.S.
submarines were at the right place at the right time, the
Japanese must have imagined that there were many more submarines
patrolling those waters than there actually were.  In early
1945, one Japanese POW testified that "it was a common saying in
Singapore that you could walk from that port to Japan on
American periscopes." (5:134)  Overall, the war against Japanese
merchant shipping played a critical part in bringing about the
defeat of Japan:
    By the end of 1944 imports of oil...had almost
    entirely ceased and domestic stocks in Japan, as
    high as 43,000,000 barrels at the end of 1941, sank
    to less than 4,000,000 by March 1945. (4:229)
More directly, the merchant war supported the U.S. island
campaigns in the Pacific since convoys often carried ground
troops to reinforce their garrisons on the Pacific islands, and
many of these were also sunk.  In one notable incident, nearly
10,000 Japanese reinforcements, bound for Leyte during the U.S.
effort to recapture the Philippines, were killed when a message
announcing their sailing was intercepted, decrypted, and passed
to Admiral Halsey who attacked, sinking the entire convoy.
    In addition to its usefulness in positioning submarines,
Ultra information derived from the maru code gave valuable
information on damage sustained by the Japanese merchant ships
in a submarine attack.  Often, after firing torpedoes, U.S.
submarines would have to dive to avoid a counterattack from the
escort ship, thereby precluding an assessment of the results of
the attack.  But when a convoy that was attacked reported the
event, including a description of losses and damage, this
information was intercepted and passed back to the submarine,
thus providing feedback.  The submarine could then reattack, if
    Exploitation of the maru code also provided some indirect
benefits.  First, it revealed the anti-submarine measures
employed by the Japanese, which led to development of
countermeasures.  Second, reports on malfunctioning U.S.
torpedoes resulted in the discovery and correction of problems
with the magnetic exploders, thus greatly enhancing the
effectiveness of submarine attacks.  Finally, the locations of
Japanese minefields were revealed, which proved to be doubly
precious.  Not only did it enable U.S. submarines to avoid
striking mines but, since Japanese ships passing through the
minefields were restricted to very narrow lanes, it was much
easier for submarines to locate and sink them.
    Another major contribution, which has already been briefly
mentioned in preceding sections, is the role COMINT played In
accurately deterimining Japanese losses through the interception
and decryption of after-action reports.  This proved to be a
vitally important function, because, without it, U.S. estimates
of losses inflicted on the Japanese tended to be grossly
    Opposing forces can be afflicted with "victory disease."
(4:83)  This is a phenomena whereby returning attackers, whether
they be pilots, cruiser division commanders, or submariners,
having viewed the battle from within the "fog of war," tend to
overestimate the damage they inflicted on their enemy.
Intercepted damage and loss reports clear that fog, but the
Japanese, without the benefit of having broken the U.S. code,
had to rely on the wildly exaggerated reports of their forces
returning from battle.  Japanese commanders believed these
reports and so thought the Americans were much weaker than was
actually the case.  The Japanese had a severe case of victory
disease, and it proved to be their undoing.
    A good example of how COMINT can prevent victory disease was
seen after a surface battle in the Solomons in 1943.  In July of
that year, a surface group of three light cruisers and four
destroyers under Rear Admiral Ainsworth was dispatched to engage
a Japanese force that was trying to land reinforcements In the
Solomons.  In a night action, Ainsworth reported that he had
sunk two enemy cruisers and "additional" destroyers which had
"vanished" from his radar screen.  However, COMINT proved that
the group had sunk only one Japanese destroyer (though another
had run aground on a reef).  There had been no Japanese cruisers
within five hundred miles of the action. (3:477)  Japanese
documents captured some time later confirmed the intelligence.
    From almost the opening day of the war, COMINT detachments,
called Mobile Radio Intelligence Units, or simply "RIUs," were
assigned to naval task force flagships to provide communications
intelligence to the embarked commanders.  This practice was
instituted in early February 1942 when an RIU sailed with Halsey
aboard ENTERPRISE, to raid Kwajalein, Wotje, and Maloelap in the
Marshalls, and continued throughout the war.  The teams
generally consisted of one language officer and four enlisted
radio operators who could copy Japanese messages.
	While shore-based COMINT stations, such as FRUPac, primarily
collected strategic intelligence, RIUs collected tactical
intelligence, monitoring the movement of ships and aircraft in
their areas by eavesdropping on Japanese tactical nets.
Additionally, RIUs collected some of the same intelligence as
the shore-based sites, but were able to get it to their afloat
commander much quicker, which was vital if the information was
to be useful in the fast-moving air warfare that had become
commonplace.  By monitoring Japanese aircraft frequencies, RIUs
could listen in on the conversations of Japanese pilots as they
approached the fleet, thereby providing "timely warnings about
whether their force had been sighted by the enemy, whether it
was in danger of imminent attack, the time and direction of the
attack, and whether interception of enemy planes had been
successful." (5:76)  Experienced radio operators could also tell
the distance of Japanese aircraft from the task force by the
strength of the signal.
    RIUs were present for every major U.S. naval operation in
the Pacific, and often were the key to success.  A few examples
will illustrate the role played by these units.
    During the previously mentioned carrier strike on the
Marshalls, the RIU on ENTERPRISE monitored the frequency
Japanese search planes were using to report to their base.  The
RIU was able to confirm the task force was undetected when it
heard a Japanese search plane report that it had "reached the
end of its patrol sector, was returning to base, and had nothing
to report." (3:362)  The American attack took the Japanese
completely by surprise and was a huge success.
    In April 1942, the aircraft carriers HORNET and ENTERPRISE
launched the "Doolittle Raid" of B-25's against the Japanese
mainland.  Surprise was critical to the success of the mission,
for if the task force was sighted before it was within range to
launch the bombers, not only would the mission likely fail, but
the task force itself would probably be destroyed.  The original
plan was for the heavy aircraft to launch from the carrier at a
range of approximately 600 miles, but when still 800 miles out,
the task force encountered Japanese picket boats.  The RIU was
able to confirm that not only had the boats reported the
presence of the carriers, but also a powerful Japanese carrier
force thought to be in the Indian Ocean had returned to the
Pacific and was likely starting a search for the American
ships.  Based on this information, Halsey launched the bombers
early and departed the area quickly, escaping without being
further detected. (2:67)  The mission was successful.
    During the Battle of the Philippine Sea, in June 1944, the
Japanese used a "master pilot" to control other attacking
aircraft during a carrier strike against the U.S. fleet
supporting the landings in the Marianas.  The RIU listened in on
the net being used by this master pilot, and so was able to feed
a constant flow of detailed information to Admiral Spruance
concerning what the Japanese were about to do next.  This action
was such a lopsided American victory that the battle became
known as the "Great Marianas Turkey Shoot." (4:256)
    In one final example, during the Okinawa campaign, the U.S.
Navy sustained heavy losses from kamikaze attacks.  Normally,
the RIUs were able to intercept no tactical signals dealing with
specific suicide attacks, though a "suicide net" was identified
and was often active when a suicide mission was in progress.
(5:118)  This advance warning enabled the fleet to be prepared
when the attack arrived.  Additionally, non-suicide attacks were
often conducted at night, presaged by late afternoon staging of
bombers from rear to forward bases.  Radio activity of this
movement was intercepted by the RIUs and provided a convenient
tip-off to the Okinawa task force.  (5:124)
    RIUs had, indeed, become invaluable, and their functions had
become integral parts of U.S. operational plans.  One carrier
Admiral wrote in 1945 that "No Task Group Commander in the Fleet
can now afford to be without an RI Unit." (5:77)
    In any endeavor as complicated as war, the outcome rarely
depends on any one event or any one group of people.  It is the
sum of all the events and all the personal efforts that lead to
victory or defeat.  So, despite the critical role COMINT played
in the Pacific War and the impact it had on the way the war was
fought, few would claim that the final outcome would have
differed had COMINT not been available.  However, that does not
mean it didn't influence that outcome.  As a noted historian
wisely observed,
    The cryptanalysts did not win the Second World War on
    their own.  But... the end came years earlier, and many
    thousands of lives were saved, because of their ability
    to read the enemy's signals. (4:17)
    Not only was Communications Intelligence not the sole
determinant of the war's final outcome, but it was also not the
sole source of intelligence.  Photographic intelligence, for
example, made an equally crucial contribution to the war
effort.  One characteristic of intelligence is that any one
source rarely provides the whole picture.  But even though
communications intelligence only contributes a part of the
picture, it is still valuable, because, as Aristotle said,
"...each adds a little to our knowledge, and from all of the
facts assembled, there arises a certain grandeur." (1:95)
1.  Colwell, Robert N.  "Intelligence and the Okinawa Battle."
        Naval War College Review, March-April 1985, pp. 81-95.
2.  Holmes, W. J.  Double-Edged Secrets.  Annapolis: Naval
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3.  Layton, Edwin T.; Pineau, Roger; and Costello, John.  And I
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6.  Van Der Rhoer, Edward.  Deadly Magic.  New York: Charles
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