In early March 1942, Japanese military planners were elated with the results of the first phase of the war in the Pacific. Almost all of the initial objectives had been achieved ahead of schedule. They had gained control of those areas of Southeast Asia and the Southwest Pacific rich in natural resources, and, at the same time, occupied strategic points surrounding those areas which would establish a strong defensive perimeter. By late February, the Japanese had brought those areas producing oil, rubber, tin and bauxite into the Greater East Asia Co-Prosperity Sphere. And they had created a defensive perimeter reaching from the Kurile Islands southeastward through Wake, Guam, the Gilbert and Marshall Islands, westward along the northern coast of New Guinea, through Borneo, Java and Sumatra up the Malaya Peninsula and again westward from Indo-China, across Siam and Burma, to the border of India.
The only objective still to be attained was the complete conquest of the Philippines. But the Allies there had been pushed back to their last refuge--Corregidor, and that was expected to fall momentarily. In the process, the Japanese had practically annihilated the British and Dutch forces in the Southwest Pacific and had dealt the United States a crippling blow by their surprise attack on Pearl Harbor.
With the first phase completed, however, the Japanese High-Command was undecided about their next objective. The Naval General Staff itself was split: one faction advocated a push westward to seize Ceylon and eventually join forces with the Germans in the Near East. Another faction wanted to isolate Australia by taking Port Moresby, on the southern coast of New Guinea, and then New Caledonia, Samoa, and Fiji. These would be bases from which they could disrupt Allied supply lines to Australia and eventually launch an invasion of Australia.
While the Naval General Staff argued the merits of these various proposals, Admiral Isoruku Yamamoto, Commander-in-Chief of the Combined Fleet and chief architect of the Pearl Harbor attack, contemplated still another operation. Admiral Yamamoto's view of the war was not always in accord with that of the other Japanese admirals. He had stated that Japan would do well for the first six months of the war, but that if it continued beyond eighteen months, he would not guarantee the final outcome. In his opinion, Japan's only hope for success lay in rapid conquests combined with the destruction of the United States fleet in the Pacific. If this were accomplished, the United States might be forced to negotiate a settlement which would recognize Japanese supremacy in Southeast Asia and the Southwest Pacific. If the United States was given time to fully mobilize her industrial potential, the outcome of the war could be in serious doubt. This was the rationale behind his decision to attack Pearl Harbor. But the American carriers had escaped the attack and were still capable of providing the United States the time it needed to mobilize her potential. For Yamamoto, the destruction of the American carriers was the number one priority.
To gain his objective, Yamamoto believed he would have to attack a position which the United States would have to defend with all available forces. And neither of the plans proposed by the Naval General Staff would accomplish this. It was unlikely that the Americans would make a last ditch stand for Australia, and certainly not for Ceylon. But they would for Midway. The Americans could not possibly allow the Japanese to take Midway because it would threaten the very existence of the United States in the Pacific. The American fleet would have to come out, and in force, to defend Midway.
Thus committed, Yamamoto would not be deterred by arguments against his Midway operation put forth by the Naval General Staff. When it seemed as if an impasse had been reached, General James Doolittle and his B-25s from the carrier Hornet bombed Tokyo. Although slight physical damage resulted from the raid, the psychological impact was enormous. The Naval General Staff, recognizing the threat posed by the continued existence of the American carriers, quickly approved the Midway operation. As a sop to the General Staff, Yamamoto lent them two carriers, the Shokaku and the Zuikaku, for their Port Moresby operation, and he added an attack on the Aleutians as part of the Midway plan. Both carriers were to be back in time for the Midway operation, but as a result of the Battle of the Coral Sea, neither would make it to Midway.
When finally completed, the Midway operation was in actuality an ambush involving the largest armada in history. The Japanese would first send a cordon of submarines to patrol between Hawaii and Midway--they would report the departure of the American fleet from Hawaii and then join the battle. Five major tactical groups were involved in the operation: the Advanced Expeditionary Force, the First Carrier Striking Force, the Midway Occupation Force, the Main Body, and the Northern (Aleutian) Force. Altogether, the Japanese would send some 200 ships and 700 planes, including eleven battleships, eight carriers, twenty-three cruisers, sixty-five destroyers and twenty submarines. The First Carrier Striking Force was to soften up Midway, which would then be taken by the Occupation Force. The Main Body, commanded by Admiral Yamamoto, would stand well to the rear. When the American fleet rushed to the defense of Midway, the Main Body, alerted by the submarines, would move into position and the desired battle would be joined. Elements of the Northern Force would then close on the flanks, and, due to overwhelming superiority of numbers, the Japanese would destroy the United States fleet.
While the Japanese debated the merits of the proposed operations, the United States Navy was trying to marshal its forces to counter the next Japanese offensive, but they did not know where or when the Japanese would strike. This was critical for two reasons: First, the United States was committed to a defensive war in the Pacific--they had to wait and react to Japanese actions, and, second, since they were committed to defend the Hawaii-Australia line with inferior numbers and weapons, the only real chance for success was to concentrate their forces at the right place at the right time.
Under these conditions, the role of intelligence became even more critical. Although a correct estimate of Japanese intentions would not guarantee the outcome of any battle, no intelligence at all, or an incorrect analysis, could result in disaster. But most of the traditional sources of intelligence--reconnaissance, prisoner interrogations, and captured documents--were denied to the Navy. The only source now available was communications intelligence. OP-20-G, the Navy Radio Intelligence Section, had the responsibility of providing communications intelligence on the Japanese Navy. Its mission was to intercept enemy radio communications, break the codes, translate the plaintext, and furnish the results to command authorities.
At the beginning of the war enemy signals were intercepted by stations at Hawaii, the Philippines, Guam and Bainbridge Island in Washington and by an extensive network of D/F (radio direction finding) stations. There were also Comint processing centers in Hawaii, Guam, and the Philippines--where intercepted communications were processed and analyzed. Later in the war a fourth processing center was established in the Main Navy Building in Washington.
Information was exchanged regularly among these processing centers. They would pass on all kinds of technical data, such as code and key recoveries, as well as intelligence. There were two categories of intelligence--Decryption Intelligence or DI, derived from the text of a message, and Traffic Intelligence or TI, derived from message externals, such as addresses, signatures, etc. Both forms of intelligence played vital roles in the battles of Coral Sea and Midway.
In March 1942, OP-20-G was experiencing serious difficulties; it had lost two of its three Pacific Island intercept stations (Guam and the Philippines), and its remaining Pacific station, in Hawaii, was critically sort of trained operators and analysts, and most of its equipment was obsolete.
As serious as all of these problems were, there seemed to be ways of surmounting them. Until full mobilization could supply more men and more and better equipment, American ingenuity would have to--and did--cope with these difficulties. But there was one problem confronting the organization which only positive results could solve. Because Comint was still relatively new and untried in wartime conditions, some of the military were skeptical of its value. Could Comint discover useful intelligence? Would it be timely? Some had little faith in Comint because they believed it had failed to provide sufficient warning of the Pearl Harbor attack. Whether those in the profession consciously realized it or not, in the minds of many they had yet to prove the value of Comint.
It has never been ascertained exactly when OP-20-G made its first step toward ultimate success by learning of the Japanese offensive directed toward Port Moresby and Tulagi, which precipitated the Battle of the Coral Sea. We do know that on March 25th the following message to Japanese units was intercepted:
All attack forces continue operations with ---- on 26th. #2 Attack Force continue to support main task and using fighters assist #5 Attack Force in the RZP campaign, and with scouts carry out patrol of your assigned area. #5 Attack Force continue attacks on RZP and ----, and carry out patrol in your assigned area.
RZP was identified as Port Moresby. In early April, both decryption intelligence and traffic intelligence revealed the nature and scope of this new Japanese offensive. Comint recorded the daily movement of planes, ships, equipment, and personnel to Rabaul in preparation for this penetration into the Coral Sea.
Then, on April 24th, OP-20-G intercepted the following:
Change #3 Truk Communication Section, for Naval call list #117, on 25 April page 5 between Kana 1 and Kana 6 insert the following in order:
Kana 1 MO Fleet
Kana 2 MO Occupation Force
Kana 3 MO Occupation Force ----
Kana 4 MO Attack Force ----
Kana 5 RZP Occupation Force
Kana 6 MO Occupation Force
Kana 7 RXB Occupation Force
Kana 8 RY Occupation Force
MO WI #---- Force of the 3rd Special Base Force
I NE #----Force of the 5th Special Base Force
MO had been determined by OP-20-G to be another designator for Port Moresby. RXB was Tulagi, and RY was still unidentified, although it was thought to be in the Gilbert Islands group. Here was a definite picture of the extent of the Japanese offensive.
Based on messages which revealed the ship departure times, OP-20-G estimated that the offensive would begin in the first week of May.
The man with the most immediate need of this intelligence was Admiral Chester W. Nimitz, Commander-in-Chief, Pacific Fleet. His intelligence officer, Lieutenant Commander Edwin Layton, was in daily contact with Commander Joseph J. Rochefort, the officer-in-charge of the Comint processing unit in Hawaii. Layton briefed Admiral Nimitz daily from decrypted intelligence reports, traffic intelligence summaries, and from whatever collateral intelligence happened to be available. Layton had standing orders to interrupt the Admiral at any time if he received urgent Comint.
By April 29th, Admiral Nimitz had been provided enough intelligence to enable him to commit his forces. On that date he dispatched the carriers Lexington and Yorktown to the Coral Sea under the command of Admiral Frank Jack Fletcher with orders to "check further advance of the enemy in the New Guinea-Solomon area by destroying enemy ships, shipping and aircraft."
Historians generally agree that the resulting battle was, tactically, a draw. The United States lost more tonnage, including the carrierLexington. The Japanese lost the carrier Shoho, the carrier Shokaku was heavily damaged, and the carrier Zuikaku, although unscathed, lost most of her planes and air crews. Strategically, it was for the Japanese the first major setback in the war. In addition, the damages and losses sustained by the carriers Shokaku and Zuikaku kept them out of the Midway operation. Thus Yamamoto would sortie to Midway with his air striking power reduced by one-third.
Apart from tactical or strategic considerations, the Battle of the Coral Sea was a significant triumph for United States communications intelligence. Comint passed its first test under fire and proved it could provide accurate, timely intelligence.
But even before the opposing forces had left the Coral Sea, a far greater challenge for Comint was already taking shape: Admiral Yamamoto was gathering his forces for the Midway operation.
Of the thousands of translated messages that indicated Japanese intentions of occupying Midway and trapping the United States fleet, less than a hundred have been found. It is, therefore, difficult to reconstruct the precise relationship between Comint and specific decisions and actions taken by Admiral Nimitz and others. However, enough evidence does exist to demonstrate convincingly that, thanks to Comint, Admiral Nimitz knew well in advance what the Japanese were planning and, as a result, he was able to turn the tables and, like a typical American western movie, ambush the enemy's ambush.
Available records indicate that until about May 11th, OP-20-G possess little hard intelligence on Japanese intentions--except for the Port Moresby operation. They did know that another campaign was scheduled to follow--possibly between May 20th and June 20th. They also new that the Japanese had created an 'A' Force and a Striking Force, probably part of the new offensive. But OP-20-G did not know the objective, the precise time of attack, or the composition of the enemy forces.
During the first week of May, traffic analysis indicated that the First, Second, and Fifth Fleets might be involved in the new operations. One intercepted message, dated May 6th, was distinctly ominous:
Because of the necessity for completing preparations for ---- operations, transfer replacement personnel for this fleet direct to indicated bases at once:
For Akagi and Shokaku to Kagoshima Base
For Kaga, Zuikaku, and Soryu to ---- Base
For Ryojo, Kiryu ----
For Ryukaku, as ordered by her commanding officer.
This list comprised almost all of Japan's major carriers, plus the light carrier, Ryujo. If all of these carriers were involved in the campaign this would be a naval force of unprecedented size!
Admiral Nimitz had 4 carriers; the Lexington and Yorktown, already fighting in the Coral Sea, and the Hornet and Enterprise, en route to the Coral Sea as reinforcements. If he were to counter this new Japanese threat, he would need additional help. Accordingly, on May 9th, he sent the following message:
||9 May 1942
||NYD Puget Sound
|Imperative Saratoga completion and readiness dates OPNAV 202100 of February be met. Request they be anticipated if practicable. Services urgently required. Advise.
During the Battle of Coral Sea and the days immediately following, Comint provided additional information about the new campaign, especially about the forces involved. Comint confirmed that Carrier Division 3 was involved, and probably Carrier Divisions 1 and 2, also that the combined army-navy landing forces assembling at Saipan would leave in late May.
But there was still no hint where all of these forces were going. On May 12th, Admiral Nimitz indicated that the Japanese were planning an operation about May 21st, involving a force of about 3 battleships and 2-4 carriers, with Oahu the possible objective. The following day two extremely significant messages were intercepted. In one, a Japanese ship requested that eight charts be sent to Saipan and held for that vessel. One chart was unidentified. The other seven covered the Hawaiian Islands area.
The second message provided the first real clue to the objective of the Japanese offensive:
The following is the schedule of the Goshu Maru--Put ashore at Imieji all the freight on board and load air hose equipment and munitions of the Imieji (seaplane unit) and proceed to Saipan by Soneka. Inform me later of your contemplated movements with Occupation Force.
The Third Air will load its base equipment and ground crews and advance to AF ground crews. Part and munitions will be loaded on the Goshu Maru as soon as that vessel arrives.
OP-20-G knew that AF was a designator used by the Japanese to represent a specific geographic location. They had recovered equivalents for a number of designators such as Port Moresby for RZP, Rabaul for R, Saipan for PS, and Oahu for AH. But thus far AF had not been identified. Because of this, differences in opinion arose, during these early days, as to where the Japanese were going to strike. Rochefort and Layton believed AF was Midway. Since they had recovered some "A" designators which equated with locations in the general vicinity of the Hawaiian Islands, they had assumed that AF was in this area. Once this assumption was made, Midway could conceivably fit. It was a strategic outpost with excellent seaplane facilities and harbor. It was probably the best target outside of Oahu itself.
Admiral Nimitz was more hesitant in his evaluation. He seemed to regard Oahu as the probable target, but he did not rule out the West Coast as a possibility.
Between May 14th and 16th, Admiral Nimitz abruptly changed his mind about Japanese intentions on the 14th, Nimitz warned Admiral King of a possible attack on Hawaii or the West Coast; two days later he told King that he expected a major landing attack against Midway.
What caused Admiral Nimitz to decide so quickly in favor of Midway? One explanation may simply be that OP-20-G provided enough decryption intelligence to convince the Admiral that AF was Midway. Another, more dramatic possibility is that, at this crucial point, the Navy, by using radio deception, tricked the Japanese into revealing the identity of AF. The exact date has never been precisely documented, but according to both Rochefort and Layton, sometime in mid-May, they approached Admiral Nimitz with a plan to proved whether or not AF was Midway. The idea was to send a message, via the cable to Midway, to the Commanding Officer of the Naval Base instructing him to:
. . . send a plain language message to Com 14 (Commandant 14th Naval District) stating in effect, that the distillation plant had suffered a serious casualty and that fresh water was urgently needed--to which Com 14 would reply, (also in plain language), that water barges would be sent, under tow, soonest . . . .
Hopefully, Japanese radio intelligence would intercept these messages and the information would then be disseminated from Tokyo in the Japanese Daily Intelligence Reports which OP-20-G would, in turn, intercept, and determine if the water situation at Midway was referenced. Admiral Nimitz agreed to the plan and the message was sent. The Japanese did pick it up and Tokyo did include, in an intelligence report, the statement that "AF is short of water."
There is no conclusive proof that this occurred between the 14th and the 16th, but, in any case, on the 16th, they intercepted the following message:
Referring to 6th Communication Units #621, as we plan to make attacks from a general northwesterly direction from N-2 to N day inclusive, please send weather three hours prior to take-off on the said day. Also, would like to be informed of enemy air activity or anything else which might be of importance. Reference to Combined Fleet #1 on the day of the attack we will endeavor to --- at a point 50 miles NW of AF and move pilots off as quickly as possible.
On May 17th, Admiral Nimitz ordered his forces in the South Pacific back to Pearl Harbor. The Yorktown was so badly damaged that it seemed unlikely that she would be available. Admiral Halsey, with the Enterprise and Hornet, was ordered to expedite his return to Pearl Harbor and to avoid detection.
The following day, Nimitz sent the following orders to a Midway-based submarine:
||CinCpac to Midway via cable
|For Cachalot x Your previous orders cancelled x Believe enemy will attack Midway using places launched from a position fifty miles northwest of Midway. Patrol that area until further orders.
The May 16th intercepted message, coupled with this one, provide the most conclusive evidence possible of Admiral Nimitz's use of Comint. He was basing orders on the information Comint provided him.
That same day, Comint confirmed the participation of Kaga, Akagi, Soryu, Hiryu, Zuikaku, and Junyo in the offensive. And in another message, this information was uncovered:
Please change the directive of the movements of the AF and AO Occupation Forces and related forces in the following manner. In accordance with ---- Operation Orders. The position in which submarines must be prior ---- will be 150 miles more or less eastward of A1.
The A1 referred to has always puzzled cryptologic historians, but it was probably a garble for AF.
Also on the 18th, Nimitz decided to reinforce Midway, station submarines off the island, and use it as a base for Army bombers; he also planned to institute searches by the long-range PBYs (patrol planes); employ Task Force 16; move out the battleships with theSaratoga; form and dispatch a North Pacific Task Force to Alaska waters; and expedite repairs on the Yorktown. All forces were to be in position by the 25th. There was as yet no definite information on the date of the attack, but Comint placed it sometime after the 25th.
During the next two days, Comint continued to piece together information about the Japanese operation. In addition to identifying more forces participating in the offensive, Comint narrowed the possibilities regarding the date. On the 20th Nimitz learned that this occupation force destined for "MI" was to depart Saipan on the 27th. Based on estimated sailing times, the attack would occur around the first of June.
It was also helpful to have MI identified. On the 20th, the Japanese sent a message, too lengthy to cite, here, in which they revised their area designators for the period covering the time of the scheduled operation. It consisted of two long columns--place names and the new code designators. Not surprisingly, the new code for AF was MI.
Comint also intercepted a message that clearly indicated the Japanese state of mind:
The next address of the 14th Air Ron will be AF.
By this time, Comint had pieced together the essentials of the Japanese operation. Admiral Nimitz knew the targets; if he did not know the name of every ships and force involved, he knew that the Japanese were using almost their entire fleet and that he would need everything he had, if he were to counter; and he knew that the attack would come sometime after June 1.
Between the 20th and the 24th, Comint continued to supply Admiral Nimitz with intelligence. Most of it confirmed what Comint had already discovered or deduced. Perhaps the most important contribution during this period was the discovery of new information on the date of the attack. By uncovering more departure dates, Comint provided information that suggested that the Midway Occupation Force and the Striking Force would arrive near Midway about the 4th of June, with the occupation scheduled for about the 6th.
By the 24th, the exact date of the attack had not been recovered, but on the 25th, Comint positively confirmed June 4th as the attack date--the result of all three Comint units working together on one message, the final operations order sent to all Japanese commanders. The way this message was solved provides an interesting illustration of the value of team work--and luck. At about 0100 on the 25th, a cryptanalyst in the Melbourne unit happened to be working on a message he had pulled, completely at random, from a rather large box of garbled traffic. He was able to recover a code group for "Midway" and one for "attack," but the message was too badly garbled for anything else. Melbourne immediately notified the units in Hawaii and Washington. Rochefort had already discovered the message but had not as yet broken it. After Washington located it, all three stations working together were able to reconstruct the message in its entirety and recover the attack plan. The date Comint gave for the attack: June 4th.
On May 28th, the Japanese changed the cipher system, and no further Comint was produced until after the Battle of Midway.
But Comint had performed superbly and Admiral Nimitz had used it to its maximum advantage in his plan to counter the Japanese operation.
On May 26th, Task Force 16 arrived in Pearl Harbor; the following day, the Yorktown limped into port. And it was learned that theYorktown could be repaired. The Navy Yard worked around the clock in a superhuman effort to put the Yorktown back into fighting condition. Task Force 16 left for Midway on the 28th, the Yorktown on the 30th.
Everything that could be done had now been done. Admiral Nimitz had deployed almost all of the resources still available to him in the defense of Midway. And he had based his commitment entirely upon Comint. Rochefort and Layton may not have had any doubts, but the last entry by a CINCPAC diarist for May 27th is poignant:
Of course it may turn out that the Japanese are pulling our leg and using radio deception on a grand scale.
But the Japanese were not pulling our leg; they were indeed coming to Midway.
Since June 1942, historians have written numerous accounts of the Battle of Midway and examined it from almost every angle. These scholars have thoroughly explored the significance of the battle relative to the outcome of the Pacific war, to the impact of carriers and naval airpower on strategy and tactics, and to the specific tactics employed by both sides during the battle. They have analyzed the command decisions and the leading personalities, and they have added accounts of heroism and courage to the histories of the naval, army, and marine units involved in the battle. Some historians have alluded to, or even attempted to show, the relationship of Comint to this battle, but these attempts have not been completely successful simply because these historians did not have access to the Comint records. As a result, although it is known that Comint did contribute to this victory, the degree of that contribution has never been fully documented, nor has the impact of the battle on the Comint profession ever been assessed.
The Comint documentation presented here is only a small fraction of that which existed at the time. But it is more than enough to demonstrate the amount, timeliness and accuracy of the Comint provided Admiral Nimitz: thanks to Comint, he knew more abut the Midway Operation than many of the Japanese officers involved in it. He knew the targets; the dates; the debarkation points of the Japanese forces and their rendezvous points at sea; he had a good idea of the composition of the Japanese forces; he knew of the plan to station a submarine cordon between Hawaii and Midway; and he knew about the planned seaplane reconnaissance of Oahu, which never took place because he prevented their refueling at French Frigate Shoals. He even knew when and where many of the forces involved would anchor on their return to Saipan.
The timeliness of the Comint was amply proven by comparing the dates of Nimitz' command decisions with the dates of the intercepted messages.
The accuracy of the information spoke for itself. There never seemed to be any doubt in the minds of the principle figures on the scene at Pearl Harbor. The only question ever raised was that expressed by the unknown diarist, when he suggested the possibility of Japanese radio deception. But Admiral Nimitz believed Comint to be accurate, and he acted on that belief. When the Japanese Striking Force appeared off Midway, on schedule, Comint was proven right.
Comint's contribution to victory in the Battle of Midway had a dramatic effect on those who used Comint and on the Comint profession itself.
For Admirals King and Nimitz and other senior military commanders, Midway clearly demonstrated the value of Comint and the ability of cryptologic professionals to function successfully under wartime conditions. it would take some time, perhaps, for this conviction to spread throughout the lower echelons of command, but for those at the top, who knew what had happened and had seen it work, there was no longer any doubt. Later in the Pacific war, it became an offense punishable by court martial for a tactical commander who had been provided Comint to disregard it.
The success at Midway established the Comint profession and gave it the recognition and respectability it needed--when it needed them the most. At the beginning of the War, Comint was not particularly high on the list of priorities in terms of the allocation of men or funds for equipment. After Midway, not only would more people be funneled into Comint, but higher criteria would be applied to the selection of potential cryptanalysts. More funds would also be made available for developing new machines tailored to the needs of the profession and for the development and installation of rapid systems of communications among the Comint units. The days of sending intercepted traffic back to Washington by air mail were gone; henceforth teletype circuits would flash D/F bearings, intercepts, technical data, and processed intelligence among the Comint units in a matter of minutes.
Another by-product of Comint's success at Midway was that new theories of joint Comint effort and consolidation suggested by some officers in the profession found support among those in a position to translate the theories into reality. Some Comint officers believed that as American forces advanced in the Pacific, intelligence production units had to advance also. Since each service had its own intelligence organization, duplication of effort and lack of coordination often wasted personnel and time. The proposed solution was the Joint Intelligence Center, where data from the field units of all the services could be processed and analyzed. Army, Navy, and Marine units would be co-located in the centers and, by close liaison and coordination, valuable time could be saved and the overall efficiency of operations improved. In 1943 this concept became a reality with the establishment of the Joint Intelligence Center, Pacific Ocean Area.
This center was not only a significant advance at the time, it was also the first real step in the creation of a truly national Comint effort, steps that led ultimately to the joint national efforts of more recent years.
The Battle of Midway has become a classic example of the successful operation of the communications intelligence process. From interception through processing and analysis, through translation to timely reporting, the entire process worked the way it was designed to work. The United States Naval victory in the Battle of Midway was a direct reflection of a truly incredible performance of the entire Naval Comint organization. As Admiral Nimitz said after the battle, "had we lacked early information of the Japanese movement, and had we been caught with Carrier Task Forces dispersed, possibly as far away as the Coral Sea, the Battle of Midway would have ended far differently."
11 May 2009