(Editor’s note: the following is the text of an address given by Jacobsen to a gathering of Naval Security Group personnel at San Diego in 2000. It has been edited slightly for clarity and to better suit this format. —RR)
Although my part in the Battle of Midway was very
small, I appreciate this opportunity to relate to you some of the more
important achievements of my contemporary naval cryptologists that made the
success of the Battle of Midway possible.
As a current member of the Naval Security Group, you can take pride in
the great accomplishments of your predecessors, not only related to the Battle
of Midway but long before World War II as well as throughout World War II.
There are not many naval cryptologic veterans alive
today that were involved in providing the communications intelligence
information that gave our inferior forces on land, sea and especially in the
air the equalizer of knowing the composition of enemy forces, and when and
where those huge Japanese forces would attack U.S. territory under Admiral
Yamamoto’s grandiose invasion plan.
This crucial communications intelligence information, when combined with
the heroic actions of fighting forces under the brilliant command of Admiral
Nimitz, led to the great U.S. victory in the Battle of Midway.
We should keep in mind that intelligence itself does
not win battles. However, I believe the
lesson of the Battle of Midway is that good, solid intelligence can make the
difference between winning and losing a crucial battle for our country. I hope you will keep this in mind in the
future.
What was the genesis of the naval cryptologic success
at the Battle of Midway? So much was
involved in building up dedicated experts in all the various fields of
cryptology that it is impossible to point to one single source. Credit must be given to many individuals who
operated under difficult conditions, extremely limited budgets, and poor
promotional opportunities. This
relatively tiny group of dedicated individuals accomplished much in their
efforts over the years to keep abreast of the growing force of the Japanese
navy and their ever increasing communications security precautions. With the Japanese instigation of war with
the U.S., this cadre of technical experts made it relatively easy to expand
into a large organization and to immediately provide increasingly vital
intelligence to not only U.S. Navy operational forces but also to U.S. Army and
Allied forces in the Southwest Pacific and Indian Ocean areas.
Despite successes with prior Japanese naval and
diplomatic codes, the high priority placed on the small group of naval
cryptologists to provide decrypts of Japanese diplomatic communications
precluded any significant decrypts of the current Japanese fleet code,
JN-25B. Immediately after the Japanese
attack on Pearl Harbor, Station HYPO in Hawaii under Commander Joseph J.
Rochefort was given the authority to attack JN-25B. By early 1942, HYPO was producing some usable JN-25B
decrypts. Station CAST at Corregidor,
which was moved to Melbourne after the Philippines fell, and Station NEGAT in
Washington soon followed with a number of important JN-25B decrypts.
HYPO first reported an offensive action in the “AK”
or Hawaiian area which culminated in the ineffectual bombing of Oahu on the
night of 4/5 March 1942. Rochefort
determined that the long range Japanese seaplane was refueled by a submarine at
the isolated island of French Frigate Shoals.
This information would later play a vital part of the preparation for
the Battle of Midway.
The Japanese geographical designator “AF” began to
appear in partially decrypted messages as early as 4 March 1942. On 13 March, Corregidor firmly identified
“AF” as Midway. Melbourne and Washington
confirmed that “AF” was Midway from subsequent decrypts, but for some
unexplained reason Washington evaluated it as a communications designator, not
a geographical designator even though Midway was obviously not a Japanese
communications station.
Decrypts in late April by Melbourne and Hawaii showed
intentions of hostile Japanese action at Dutch Harbor and Kodiak in the Alaskan
area.
Beginning on 1 May, activity in Japan proper
reflected preparations for both the Midway and Alaskan areas and provided
detail of Japanese planning and the size of the forces committed to each
objective. As the Japanese ships
departed their anchorages, communications intelligence provided information on
their future disposition. Both
Melbourne and Hawaii reported the pairing of Japanese Carrier Divisions 1 and 2
for exercise activity in home waters on 3 and 12 May. In addition, HYPO provided a decrypted message of 7 May 1942
containing the complete agenda for an “aviation conference” on 16 May called by
Vice Admiral Nagumo in Kagoshima, Kyushu.
Also to be discussed was an “amphibious assault” and battle for “air
superiority” together with a study of organizations for use in dive bombing,
torpedo attacks, bombing, and strafing to wipe out local resistance.
For some time the status of Admiral Kondo’s powerful
Second Fleet was clouded. Finally on 8 May 1942, HYPO correctly
associated the carriers of the 1st Fleet with several important 2nd Fleet
elements and warned of a possible creation of a strike force organization under
Vice Admiral Nagumo, Commander 1st Air Fleet, consisting of CarDivs 1 and 2,
CruDiv 8, two battleships from BatDiv 3, and other 2nd Fleet elements. These early correct conclusions gave a major
advantage to the planners in the U.S. Pacific Fleet. They were reinforced
by Melbourne on 9 May by a decrypt ordering destroyer screens for many of the
capital ships in the Striking Force and revealing a sailing date from Sasebo of
21 May.
On 14 May Admiral King directed Admiral Nimitz to
declare a state of “Fleet Opposed Invasion” and gave Nimitz complete control of
all military forces, including B-17s in the Hawaiian Islands. By 16 May Admirals King and Nimitz were in
almost total agreement concerning Japanese intentions toward Midway and the
Aleutians. However, this view was in
sharp contrast to the confusion that reigned between OP-20-G (Station NEGAT)
and War Plans staff under Admiral Richmond K. Turner. Turner placed some
ridiculous restrictions on what Station NEGAT could report.
On 16 May, Nimitz ordered Admiral Halsey [Task Force
16 with USS Enterprise and Hornet] to return to Hawaii,
indicating the Japanese would probably make simultaneous offensives against
Port Moresby, Dutch Harbor, and Midway where the main striking force would be
employed.
Two days later, all three navy cryptologic centers
reported that the Strike Force’s attack would be from the northwest from N
minus 2 days until N day, while Hawaii and Melbourne added that the attack
would be launched from fifty miles northwest of AF. While this did not solve the attack timing problem completely,
Nimitz immediately sent messages to Halsey and Fletcher [Task Force 17 with USS
Yorktown] to expedite their return to Pearl Harbor as well as ordering
submarine search activity off Midway to an area fifty miles northwest of the
island.
An acrimonious relationship between Admiral Turner
and his War Plans Division and OP-20-G continued, with Turner directing
Commander Redman not to comment on certain intelligence evaluations and assume
that Turner’s views were correct. The
record suggests that the analysts in War Plans and OP-20-G were so engrossed in
their own activities that they sometimes overlooked information concerning the
Imperial Fleet readily available from translations in OP-20-GZ and the daily
reports of the Pacific centers.
While the Pacific centers were convinced that the
identity of AF was Midway because of its position in the “A” or American
digraphs in the Japanese designator system, various persons at OP-20-G and in
Washington thought it might be Johnston Island, Samoa, the U.S. West Coast or
even Hawaii itself. HYPO was aware of
this lack of agreement on AF in Washington.
In order to rid themselves of this annoying backbiting, Rochefort
approved a ruse that was probably thought up by Jasper Holmes, the author of Double
Edged Secrets. Nimitz approved the
message to be sent in the clear from Midway complaining of a water
shortage. Rochefort let Melbourne make
the first report of the decrypt from Tokyo Naval Intelligence advising of a
“water shortage at AF.” Even the
naysayers in Washington could not argue with this confirming evidence.
Additional information about a Japanese northern force
prompted Nimitz to activate Task Force 8 under Admiral Robert A. Theobald. In spite of accompanying and subsequent
accurate information about Japanese intentions in the Aleutians from decrypts,
Theobald chose to treat such information as enemy deception and moved his
forces out of the area to the Kodiak vicinity.
That allowed the enemy to pound Dutch Harbor and occupy Kiska and
Attu.
From information of Japanese successes in determining
carrier movements simply by monitoring air to ground communications, Nimitz
ordered Halsey and Fletcher to maintain radio silence, particularly among the
aircraft when coming in to land. He
also warned MacArthur that the Japanese were intercepting air-to-ground
contacts between Port Moresby and allied planes. Nimitz also implemented a MacArthur suggestion that two or three
U.S. vessels in the South Pacific conduct radio deception to create the
impression that our carriers were remained in that area.
On 22 May, a Melbourne decrypt revealed the word
“Midway” in a request for photographs of the island that had been “handed over
to you.” Washington published a message
from Nagumo to the 11th Air Fleet showing that his carriers had 33 aircraft on
board that were destined to be the nucleus of land based aircraft in the new
Japanese perimeter. Their loss was
completely unnoted in accounts of Japanese carrier losses.
The 25th of May began with HYPO’s critical discovery
of the Japanese date cipher. Now the
U.S. possessed the means to determine the final ingredient of the Japanese
plans—when the attack would take place.
Application of this information allowed Rochefort to predict that the
Japanese attack on the Aleutians would occur on 3 June and on Midway on 4 June. Despite objections from his staff, Nimitz
decided to base his final timetable on these dates. Melbourne applied this date cipher information to older traffic
and alerted the Pacific Fleet that on the 22nd of May CruDiv 8 and the
battleships Kongo and Kirishima were scheduled to depart the
Inland Sea of Japan.
Task Force 16 (Hornet and Enterprise)
under Admiral Halsey returned to Pearl on the 26th and began a whirlwind of
preparation for battle. The CINCPAC
Bulletin of the 26th reported that the Northern Force had begun to depart Ominato
and that all the Japanese carriers were probably at sea. Admiral Nimitz advised King how much he was
dependent on communications intelligence and noted that they were only copying
60 percent of Japanese naval messages and only decrypting 40 percent of those
copied. King attributed all of the
Navy’s progress in the Pacific to the success it was having from timely
information from Japanese naval codes.
Without this information King said, “disaster is probable.”
On the 27th of May, the Yorktown finally limped into
port, showing the damage inflicted during the Coral Sea battle. This good news was offset by some bad news
from Commander Rochefort’s center: a
new underlying code (JN-25C) and additive cipher had been introduced that
rendered unreadable almost all the texts of JN-25 messages from the 27th
on. However, some previously originated
messages were still readable including one from the 5th Fleet that contained
tactical call signs for the Northern Force, its Strike Force, and the
Occupation force for “AQ” and “AO” identified as Kiska and probably Attu. Again, Theobald refused to believe this
intelligence and kept his force near Kodiak.
Another prior message concerned the “Ichiki Detachment” to command the
2nd Combined Landing Force, which was to occupy Midway’s Eastern Island. A third message revealed the intended use of
civilian engineers captured on Wake Island to be used in the rebuilding of
Midway. Additional warnings that the
carriers were at sea were also published.
On 30 May, U.S. task force commanders were alerted by
HYPO that direction finding had located three submarines in northern waters and
one west of Midway. That day, the Yorktown
(Task Force 17) slipped out of Pearl but was detected by the ComInt unit aboard
the Yamato, Admiral Yamamoto’s flagship. However, due to radio silence restrictions, this information was
not passed on to the Japanese carriers.
NEGAT in Washington reported that the carrier Ryujo was at sea
with the Northern Forces and that the Commander of the 6th Army Air Force was
probably aboard the Akagi.
An old message produced the important information
that fighter pilots from the carrier Zuikaku had been transferred to the
Northern Force, ruling out the possibility that the Zuikaku could be
called on to support either the Aleutian or Midway campaigns. Another message determined that major
participants were called to a conference aboard the Akagi on the 26th,
which meant they were still in port on that date.
Melbourne’s analysis of air activity in the Marshalls
on 2 June led them to conclude that the Occupation Force was approaching the
Marshalls. However, Admiral King’s
headquarters report of that day contained serious errors. It estimated that BatDivs 2 and 1, CarDiv 4,
and DesRon 3, parts of the Main Body, were still in the Bonins home waters area
when in fact this force was approaching the western edge of the occluded front
northwest of Midway. Perhaps, more
importantly, the Office of Naval Intelligence chose this moment to report the
presence of a fifth carrier, and identified the carrier as the Zuikaku. Fortunately, Admiral Nimitz and his
intelligence staff had confidence in the information being generated by the
centers in the Pacific, and this ONI estimate was not acted on or repeated to
the task forces off Midway.
As predicted by HYPO, the Japanese offensive against
the Aleutians began on 3 June with the carriers attacking Dutch Harbor. Shortly thereafter, Midway notified Nimitz that
the Japanese “Main Body” was sighted at 2100Z by a patrol plane bearing 261
degrees and a distance of 700 miles from Midway. After a second sighting of a smaller group of warships and cargo
vessels, Nimitz advised that the forces sighted were the attack and occupation
forces, not the main body. HYPO’s
report of 3 June identified Admiral Yamamoto, CINC of the Combined Fleet as in
overall command and correctly identified major commanders and functions of 2nd
Fleet, 1st Air Fleet, and 5th Fleet.
Just after midnight on the morning of 4 June, Nimitz
realized he had not yet advised the task forces how far the “Main Body” was
from Midway. In addition to repeating
earlier reports on its course and speed, he concluded it was now 574 miles from
Midway. At 0604 Midway time, a
reconnaissance plane from Midway spotted two Japanese carriers and their
escorts and reported “many planes heading Midway” from 320 degrees, distance
150 miles. Less than a half hour later, Midway was attacked by Japanese
carrier aircraft.
Nimitz was only able to muster 47 warships and 26
submarines against the Japanese fleet of 113 warships and 16 submarines. However, the U.S. was able to concentrate
its forces at Midway with a slight advantage at the scene of the battle with
three carriers, 22 escorts, 234 aircraft afloat and 110 at Midway versus four
carriers, 17 escorts, 229 aircraft and 17 seaplanes for the Japanese. In addition, Admiral Nimitz and his task
force commanders had advance knowledge of the identity of the Japanese objectives;
virtually the entire Japanese Midway and Aleutian order of battle; the
organization of the Midway forces into a Striking Force, Occupation Force,
Invasion Force; the preliminary and final timetables of the Midway and Aleutian
Striking Forces; the general direction from which each force would approach
Midway, and the Midway Strike Force’s plan of attack. All of that information was supplied by communications
intelligence in time to influence decisively the provisions of Admiral Nimitz’s
Operation Plan 29-42.
In addition, luck was on the side of American forces
in several key instances. Partly due to
poor Midway bomber group sighting reports, two of the U.S. carrier aircraft
groups [from Enterprise and Yorktown] were fortunate to locate
the enemy carriers after changing their original course, while Hornet’s
planes failed to make any contact. The
late takeoff of the #4 search plane from the cruiser Tone prevented the
Japanese from discovering the presence of U.S. carriers in time to make
significant operational changes.
The Americans lost only one carrier, one destroyer
and 147 planes, while the enemy suffered the loss of four large carriers, all
their aircraft, as well as one heavy cruiser and the damage to one heavy
cruiser. These losses plus the
rejection of the enemy invasion and occupation forces resulted in a huge
victory for the U.S. Navy early in WWII.
This great success after so much bad news from Pearl Harbor, the
Philippines, and Southeast Asia was a great morale booster to the American
people.
After the battles of Coral Sea, Midway and the Aleutians, the invaluable contributions made by communications intelligence were recognized by senior naval officials in Washington and Honolulu. In their words, communications intelligence had given the United States a “priceless advantage” over the Japanese. In few battles before or since would any navy possess an enemy’s order of battle, their plan of attack, and their timetable, all of which had been provided to the U.S. Navy’s high command by the communications intelligence units in Hawaii and Australia under the direction of Commander Joseph J. Rochefort and Lieutenant Rudolph Fabian, respectively.