The first indication that the Japanese Combined Fleet might return to the Central Pacific, after its support of operations around the Philippines, Malaya and the Dutch East Indies in December through February, came on 5 March, not through signals intelligence but because of a minor Japanese bombing attack on Hawaii. The Americans correctly concluded that the attack had been launched from the Marshall Islands, via a refuelling stop at an isolated oceanic anchorage known as French Frigate Shoals. What was important for the future about the 5 March raid was that the American intercept stations were able to identify it with something denoted in Japanese codes as the K operation. The significance of K became more apparent on 6 May, when HYPO, the Hawaiian station of American cryptanalytic intelligence, deduced that K was part of an encrypted geographical designator standing for Hawaii. The American codebreakers were beginning to recognise that the Japanese used three (later two) letter groups to denote geographical objectives, those in the American zone of operations beginning with A (hence AK for Hawaii), those in the British zone beginning with D and those in the Australian zone with R.11
This was an important breakthrough but its value was set back by the introduction of new security measures in the Japanese fleet, consisting mainly of a change of call signs between ships but also between shore and ship. The changes greatly complicated the codebreakers’ ability to identify the location of individual ships and the composition of fleets.12 Their difficulties were compounded by the retreat of the main Japanese naval units into radio silence, as before Pearl Harbor. On 13 March the American cryptanalysts broke into the main naval code, JN-25, but that success was shortly negated by the Japanese adoption of a new system of cipher additives to the code groups. Just before the change the Americans got a crucial insight into Japanese intentions via a request from an unidentified ship for a supply of charts, clearly indicating the interest of the Japanese fleet in the Hawaiian group of islands and its western outliers, which included Midway. Nimitz, commanding the U.S. Pacific Fleet, accordingly concluded that he was faced by four possible Japanese operations: an attack on Midway-Hawaii; an attack on the Aleutian Islands, outliers of American Alaska; an attack on other Central Pacific islands; a renewed attack on the islands of New Guinea, any of those operations to be launched between 25 May and 15 June.13
As a result of the Japanese adoption of the new cipher additives to their basic code, JN-25, the result being known to the Americans as JN-25B or Baker, the American cryptographers lost their way into Japanese transmissions during May; or should have done. Because, however, of the difficulty the Japanese found in distributing new additive books across their enormous area of conquest, individual transmitters made the mistake, which may be called a classic mistake so often has it betrayed encryptors, of transmitting messages in both the old and the new code, to ensure accuracy of reception. The Americans, able to read the old code, were thereby enabled to read some of the new, so that by late May they had established the outline of their enemy’s developing plan.
The composition of the attacking force was the first crucial matter to be discovered, though largely by traffic analysis—identifying individual ship call signs and detecting their location—rather than cryptanalysis. On 17 May, Admiral King was able to publish, to a limited circle, an assessment of the strength available to the enemy for what was now believed—but not confirmed—to be an offensive against Midway and the Aleutians. For Midway, it consisted of four fast battleships, two cruiser divisions, two carrier divisions with a fifth carrier attached, two destroyer squadrons and a landing force; for the Aleutians, a cruiser division, a carrier division, formed of the two old, small carriers Ryujo and Zuiho, two destroyer squadrons and a landing force.
Next day, 18 May, King was able to narrow the geographical frame. A message was intercepted from Admiral Nagumo, commander of the Combined Fleet, reading, “since we plan to make attacks roughly from the northwest from N minus 2 days until N Day request you furnish us with weather reports three hours prior to the time of take-off on said days.” From parallel messages intercepted by the decrypting centres in Melbourne (CAST) and Hawaii it was discovered that the Japanese aircraft for which the weather forecasts were intended would be launched “fifty miles northwest of AF.”14
AF was one of the indicators used in Japanese encoded messages to indicate geographical locations. So at least some of the American cryptanalysts believed; unfortunately, others thought otherwise. Commander Edwin Layton, the extremely efficient Fleet Intelligence Officer of the Pacific Fleet, took the view that AF was a geographical indicator and so informed Admiral King in Washington; he specified Midway and Hawaii as the targets of the forthcoming Japanese offensive and Saigon and Ominato, in the Japanese home islands, as the strike forces’ departure points. This assessment arrived at a moment when the various officers of NEGAT (as OP-20-G was code-named) were deep in argument with Admiral Richard Turner’s War Plans Division over what portended in the Pacific. The argument, as so often occurs in bureaucracies, took on a life of its own, separate from the realities of the outside world. OP-20-G had become divided into three sub-branches, OP-20-G1 (Combat Intelligence), OP-20-GZ (Translation) and OP-20-GY (Cryptanalysis). Their counterparts in War Plans began to disagree with the intelligence experts over detail, until a full-scale office war was in progress. Turner, head of War Plans, and Redman, head of Naval Communications, responsible for naval intelligence, eventually came to daggers drawn over the issue of whether the Japanese commander of Fifth Fleet “is to command any force now concentrating in Northern Empire Waters”; Turner, who was senior to Redman, directed him “to assume that Admiral Turner’s views are correct.” Turner, wrongly, took the view that the Japanese offensive would be a continuation of the Coral Sea campaign, Redman that it was directed at AF, which could not be New Guinea.
This at a time when a huge Japanese fleet was gathering to threaten the United States’ last centre of power in the Pacific, Hawaii and its nearby islands, with a potentially devastating attack. What follows cannot be documented, since evidence is lacking. It seems probable, however, that at a local level, HYPO (Hawaii) took steps to resolve the issue unarguably. An undersea cable link, secure from Japanese ears, still operated between Hawaii and Midway, 1,300 miles to the west. The idea of using it deceptively has been attributed to Captain Joseph Rochefort, HYPO’s energetic chief. The story goes that on 18 or 19 May, with Admiral Nimitz’s permission, a cable message was sent from Pearl Harbor to Midway instructing the garrison of the tiny island to report a water shortage, signalling by radio in plain language. On 22 May CAST at Melbourne reported the interception of a message from Japanese naval intelligence in Tokyo (KIMIHI) reading as follows: “The AF air unit sent the following message to [Pearl Harbor] on [May] 20]. ‘Refer this unit’s report dated 19th, at present time we have only enough water for two weeks. Please supply us immediately.’ “ CAST appended, “Have requested [Pearl Harbor] to check this message—if authentic it will confirm identity ‘AF’ as Midway.”15
The objective of the coming Japanese offensive was now known: Midway. Subsidiary decrypts identified the Aleutians as a secondary objective; as they were ungarrisoned, that threat could be ignored, even though the islands were sovereign American territory.
Finally, on 25 May, HYPO broke the Japanese navy’s date cipher. By applying the decrypts to old intercepts, Rochefort in Hawaii was able to establish that the attack on the Aleutians would begin on 3 June, the offensive against Midway on the 4th. Nimitz, who reposed great faith in his intelligence decryptors and analysts, accordingly called forward his forces to meet the threat. TF 16 (Hornet and Enterprise) was recalled to Pearl Harbor on 26 May, to prepare for battle. TF 17 (Yorktown) was already there, repairing the damage suffered at Coral Sea. He also positioned a submarine screen northwest of Midway to detect the approach of a Japanese strike force.16
General MacArthur, commanding in the southwest Pacific, lent important assistance at this stage to Nimitz’s countermeasures, by recommending that radio deceptive measures be instigated, to suggest to the Japanese that a carrier group was still in the Coral Sea. Nimitz agreed, and accordingly the cruiser Salt Lake City and the seaplane tender Tangier steamed due south of New Guinea exchanging radio traffic that simulated carrier transmissions.
It was fortunate, nonetheless, that OP-20-G and its outstations had, by whatever means, established in late May what the Japanese intended, for the intelligence climate then turned against the Americans. The Japanese relapsed into radio silence, as they had done before Pearl Harbor, while their own listening services, on the alert for any American reaction to the despatch of the Midway strike force from the Inland Sea, began to report a significant increase in what it identified as “Urgent” messages from Pearl Harbor. Japanese intelligence also noted the sighting of American patrol aircraft far west of Midway and the interception of messages from an American patrol submarine in the path of the Midway landing fleet. For inexplicable reasons, Admiral Yamamoto withheld the information from the Midway strike force. It may have been that he did not wish the force to break the radio silence imposed on it by requesting clarification; the result, whatever the motive, was that Admiral Nagumo and his carriers steamed on towards Midway in ignorance of a gathering American riposte.17
The complexity of the intelligence plot in the first days of June defies easy exposition. Over the enormous expanse of the Pacific Ocean and its surrounding coastlands, the Japanese appeared to be unchallengeably in the ascendant. Indo-China, thanks to the complaisance of the French Vichy government, was under their control. Coastal China was under Japanese occupation. British Malaya and Burma had just fallen into Japanese hands. British India and Ceylon, with the surrounding waters of the Indian Ocean, were under threat of invasion, following vigorous naval attack. The Dutch East Indies had been occupied, together with most of Australian New Guinea and its outlying islands. The Central Pacific islands, mandated to Japan after 1918 or captured from the Americans and British in the first months of the current war, were Japanese oceanic strongholds. Australia itself, whose Northern Territories had already been bombed by the Japanese, was in a state of defence against invasion. The American-protected Philippines had just surrendered. All that remained to Japan’s enemies as points of resistance to what seemed its inexorable advance to Pacific domination were the American Hawaiian archipelago and its outlying island of Midway.