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Battle of Midway


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Battle of Midway: 4-7 June 1942: The Role of COMINT in the Battle of Midway (SRH-230)

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.

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