During the long night, the eve of one of the great carrier battles of the Solomons Campaign, radio intelligence monitors on board the Japanese aircraft carrier Shokaku overheard the enemy’s coded transmissions. They could not read the messages but these took the format of contact reports, and the signal strength was very high, indicating a nearby emitter. The inevitable conclusion was that Allied search planes had sighted the Japanese task force and were reporting its presence. Imperial Navy carrier commander Vice Admiral Chuichi Nagumo had been in a spirited debate with his chief of staff over whether the fleet should head toward Guadalcanal to smite Allied flotillas they assumed to be in waters off the Solomon Islands. Chief of Staff Rear Admiral Ryunosuke Kusaka advised caution.
The radio intercept ended the dispute. Nagumo ordered the fleet to turn north, and his carriers steamed away from the Americans—who were, indeed, out there—during the predawn hours. As a result, at the Battle of Santa Cruz, which took place that day, the Imperial Navy carriers would be positioned far enough from the American ones to reduce the effectiveness of U.S. strike aircraft. Some of the American bombs loosed over Nagumo’s carriers that day would be dropped by the scout planes that re-established contact with the Japanese early on 26 October 1942. The rest came from a single one of the U.S. strikes, the only one to reach Nagumo’s position. That became a basis for a significant Imperial Navy victory at Santa Cruz, and the groundwork had been laid by Japanese intelligence.
‘But a Fraction of the Story’
This anecdote from the Santa Cruz action is but a fraction of the story of what was probably the outstanding intelligence success for the Japanese during this phase of the war. In the weeks leading to the battle, Tokyo’s forces built up a fragmentary but serviceable picture of what the opposition might look like. Radio monitors warned of an aircraft carrier unit—which turned out to be the task force built around the USS Enterprise (CV-6)—leaving Pearl Harbor. Navigators could calculate from that the moment when she might appear in the South Pacific. Three weeks before the engagement, the Japanese obtained a copy of the low-level aircraft code used by the Americans, taken from the wreck of a torpedo bomber shot down attacking a Japanese base in the upper Solomons. On 13 October an Imperial Navy floatplane scout made the first actual sighting of an Allied force at sea, although it contained only battleships and other heavy surface vessels. Those sightings were confirmed by submarines, one of which managed to torpedo the American cruiser USS Chester (CA-27).
Other subs, specially equipped for scouting by carrying their own floatplanes, sent out reconnaissance flights over both the main Allied naval bases in the South Pacific. The numerous air searches from the Japanese main base at Rabaul, and by the Nagumo fleet itself, detected American carriers twice during the fortnight preceding the Santa Cruz battle. Even fruitless searches said something about where the Allied fleet was not. And on Guadalcanal, the Imperial Navy managed to install a radio direction-finding (RDF) unit, complementing RDFs already in service at Rabaul, leading to numerous notices of Allied shipping sent over the radio networks serving Japanese intelligence. At Santa Cruz the Nagumo fleet would have to find the American carrier force to attack it, but the intelligence was good enough that the Japanese knew to look for it.
‘Focused on the Allied Side’
Discussions of the importance of intelligence in the Pacific war have largely focused on the Allied side. The codebreakers get the major attention, whether for their breaking of the Japanese diplomatic codes in “Magic,” the daily round of “Ultra” decrypts of Japanese naval messages, or the knowledge gleaned from Tokyo’s naval attachés abroad—most prominently Admiral Hiroshi Oshima in Berlin—through reading attaché ciphers. Work with Japanese naval codes, which Americans knew as JN-25, led to the incredible victory at Midway in June 1942, and the signal success of the shoot-down of Japanese commander-in-chief Admiral Isoroku Yamamoto in April 1943. Also garnering attention has been Allied photo reconnaissance and the work intelligence did in translating captured documents or interrogating prisoners. At Santa Cruz specifically, the American mobile radio units on board the carriers Hornet (CV-8) and Enterprise furnished explicit warning of the approach of the Imperial Navy’s air strikes and in at least one case were able to discern that an attack formation consisted of torpedo planes, not dive-bombers, crucial to combat air controllers in selecting the altitude at which to station defending fighter interceptors.
The Japanese also had their successes, with Santa Cruz not being the only one. In the very first Solomons naval action, the Battle of Savo Island on 8 August 1942, a mobile radio unit with the Japanese commander, Vice Admiral Gunichi Mikawa, detected emissions characteristic of U.S. carrier operations, alerting him to the nearby presence of Vice Admiral Frank Jack Fletcher’s task force. Knowledge of the potential intervention of American carriers became a factor in Mikawa’s decision to withdraw after his initial foray, which crippled a strong Allied cruiser flotilla. In late August, at the Battle of the Eastern Solomons, the Imperial Navy used radio deception techniques to fool the Americans about the presence of Japanese carriers. At Santa Cruz itself, Japanese radiomen spoke up on American aircraft radio circuits, giving out phony information to mislead American pilots.
Several weeks later, when Japanese Admiral Nobutake Kondo was headed for Guadalcanal to execute a battleship bombardment, his mobile radio unit intercepted an American submarine contact report that alerted Kondo to expect opposition. The subsequent miscarriage of Kondo’s mission in what became the “Battleship Action” of the Naval Battle of Guadalcanal was no fault of the “special duty” group, which is what Japanese called their mobile radio units. Indeed, toward the end of November a special duty group warned Tokyo Express commander Rear Admiral Raizo Tanaka that he had been detected, laying the basis for a signal Japanese victory at the Battle of Tassafaronga.
Attention to Japanese Intelligence
Forests of trees have been consumed in studies of Allied intelligence triumphs. By contrast, the Japanese side has received little attention, with the exception of Tokyo’s prewar efforts against the United States in general and its work to obtain information for the Pearl Harbor attack. Most of the rest of the thin stack of studies on Japanese intelligence focus on the Imperial Army and its intelligence for land warfare. Coverage of the wartime activities of the Imperial Japanese Navy has been negligible. Yet there is a story to be told on the Japanese side, as this narrative indicates, while the distinctions between the intelligence efforts of the adversaries, in themselves, help explain Allied victory in the Pacific. Illuminating Japanese naval intelligence is the purpose here. To keep the subject manageable it will examine Imperial Navy activities during the Solomons Campaign that began with the Allied landing at Guadalcanal in August 1942 and ended with American–New Zealand invasions of Bougainville and nearby islands in November 1943.
On the Japanese side, intelligence for the Pacific war was the responsibility of the Third Bureau (joho kyoku) of the Naval General Staff (NGS). Like its U.S. counterpart, the Office of Naval Intelligence (ONI), the Third Bureau served a crucial function and had been created as early as 1896. But Japan placed less importance on its efforts in this field. The staff at NGS headquarters comprised just 29 officers. Only one senior man and a few assistants could therefore be allotted to the Bureau’s 5th Section, responsible for intelligence on both North and South America. By comparison, ONI had some 230 people on staff in Washington alone, with a total complement of about 2,200 in 1941. And this does not take into account the substantial manpower of the U.S. naval codebreaking operation. For the British, the Royal Navy’s Intelligence Division had 161 staff at headquarters, again far outnumbering the Japanese. Including all Japanese naval-intelligence personnel, among them staffs with fleets and units, field organizations, attachés, and the special duty groups, the total on the Japanese side amounted to only a fraction of those with ONI alone.
The Third Bureau ran a network of naval attachés throughout the world, provided expertise to the fleet, and was supported by an array of specialist units, most notably the Owada Communications Group. From before the Midway battle until the eve of the Japanese defeat on Guadalcanal and their evacuation from there, the Third Bureau was led by Captain Kanji Ogawa. A long-service intelligence expert who had masterminded Japan’s espionage for Pearl Harbor—as naval attaché in the United States and then chief of the 5th Section—Ogawa would be promoted to rear admiral and bucked upstairs to the Cabinet unit that studied the overall war situation. His successor from the end of 1942 would be Captain (then Rear Admiral) Hideo Yano, who had skippered the battleship Nagato when she was fleet flagship.
‘Unsinkable Bases’
Captain Yano, a Navy Ministry apparatchik who had specialized in armaments, had been immediately behind Ogawa on the Navy List. He lacked Ogawa’s extensive intelligence experience. In January 1943, with Japan about to withdraw from Guadalcanal, Yano told a German reporter in Tokyo that the Americans were wrong to believe they could capture any island in the Pacific if given enough aircraft carriers, because those islands were unsinkable bases for Japan’s air force. The truth was that Japan’s aircraft production was faltering, and Yano, an expert in that field, ought to have known better.
Chief of the American section throughout the Solomons campaign was Captain Kaoru Takeuchi. Colleagues considered Takeuchi level-headed and conservative. He had had a well-rounded career afloat, with fleets, and at the Naval General Staff, and in the final prewar months he had spent in the Mandated Islands. Takeuchi also had a deep intelligence background, having served his initial tour with the 5th Section as early as 1932. He was something of an expert on the United States, having been naval attaché in Ottawa in 1936–37, later an instructor on American history at the Japanese Naval War College, and came to the 5th Section from a job at the same institute where Ogawa would subsequently be posted. In fact, Takeuchi’s expertise was considered so important that, while heading the American section, he was given additional duty back at the War College, lecturing on the United States.
Information from Cities Worldwide
A network of attachés furnished some of the Imperial Navy’s vital data. Probably the most important were those in Berlin and Rome, where the Japanese traded information with their German and Italian allies; Stockholm, where the Imperial Navy received American publications (the Japanese were avid readers of The New York Times); Lisbon, a spy haven for all sides; and Mexico City, a location from which the Japanese could keep watch on the Panama Canal and attempt to track American warships transiting from the Atlantic to the Pacific oceans.
The Owada Communications Group, technically known as the Central Radio Intelligence Group, was the special-duty unit that matched the radio interception efforts of the U.S. Navy. For cover purposes the Owada Group formed a section of the Communications Bureau of the Naval General Staff. Located about 20 miles from Tokyo, couriers carried radio intercepts from Owada to the Bureau’s 2nd Section (tokumu han), where specialists put radio information into a form it could be used best by intelligence. The most urgent items passed over the telephone.
Captains Ogawa, then Takeuchi, served as ex-officio chiefs of the special-duty groups. Its technical director was Captain Fujimasa Ushio, who had been comrades at the Eta Jima naval academy with the skippers of two of the Japanese carriers sunk at Midway. Ushio burned to get back at the Americans. By 1938 the Owada Group already employed 118 radio receivers, plus 20 direction finders, supplementing its home installation with satellite stations at Ominato, Wakkanai, and Maizuru in the Home Islands, Chinkai (today Jinhei) in Korea, Takao in Taiwan, and Marcus Island in the broad Pacific.
The naval attaché’s office in Mexico added a radio monitoring unit in 1940. In fact the attaché from that time, Commander Tsunezo Wachi, had been a special-duty group member. The Imperial Navy’s 1st Combined Communications Unit, and then its 4th Communications Unit in the Mandates, and 8th Communications Unit at Rabaul, hosted special-duty detachments to monitor Allied transmissions. At sea the 25th Communications Unit with the Combined Fleet and the 2nd Communications Unit with the Second Fleet provided the mobile radio units for special duty.
‘Mixed Success’
Captain Ushio and his codebreakers had mixed success with their radio-intelligence efforts. By early 1942 Owada was detecting and reporting new radio call signs for Allied warships within 15 hours of first use. The Japanese were very good at traffic analysis. They tracked the length and numbers of messages from various emitters. They tabulated message traffic by call sign, by volume, by code type, by priority, by signal procedures used, and by whether messages were sent unencoded. Message traffic was graphed against time of day for every sector of the Pacific. Traffic analysis told the Japanese much.
Code work proved more difficult. At peak, only 90 to 100 men—a fifth of the Owada Group’s personnel—actually labored to break codes, backstopped at the 2nd Section of the Communications Bureau at NGS. Owada specialists were almost always able to recover the times and addressees for messages even where the contents resisted decryption. The codebreakers had repeated successes with Allied weather and merchant-shipping codes, though much less with Allied higher-level, or fleet codes, except in instances where codebooks were captured, such as that taken from the American airplane in the Solomons in October 1942.
The Japanese high command, styled Imperial General Headquarters, negotiated “central agreements” for major operations. This device preserved the independence of the Imperial Navy and Japanese Army. Use of the procedure extended into the field of intelligence. The agreement the Japanese executed for the South Pacific made the Imperial Navy responsible for intelligence in the Solomons while the Army took the lead in New Guinea. This did not prevent the other service from contributing data—for example, the first aerial photos to help the Japanese plan their invasion of Rabaul were taken by Army officers who went along on an Imperial Navy scout flight; or again, in January 1943, when the Army contributed an accurate estimate of the American forces then on Guadalcanal—but the bulk of the task was carried out under the agreement. In the spring of 1942 it was therefore an Imperial Navy base force that issued the directive governing occupation, administration, and intelligence in the Solomons.
Same Principles, Different Results
Japanese intelligence employed principles identical to Allied intelligence. Information on topography and local developments was obtained from indigenous tribesmen. Combat patrols also gathered information. The Japanese collected and exploited captured documents. They recognized the value of interrogating prisoners—although the Japanese penchant for executing captives conflicted with that purpose. (Early in the Guadalcanal battle, the Japanese missed a crucial opportunity to discover the Allies’ success at breaking their naval code when, having captured the intelligence officer of the 1st Marine Division, Lieutenant Colonel Frank Goettge, who was aware of this secret, the Japanese executed him out of hand.) Tokyo developed and implemented a propaganda strategy. They closely followed Allied commercial broadcasting, kept special-duty groups on the watch, and employed aerial photography. Intelligence officers paid attention to movements and strength of Allied air units, status and equipment of airfields, disposition of warships and supply forces, unit identifications, and Allied signals. The Japanese graded information, too (“undoubtedly reliable,” “probably reliable,” “authenticity is undetermined”). Any Allied intelligence officer would have recognized these collection categories and procedures.
The Imperial Navy was to set up its own coastwatcher posts—similar in concept to the famous Australian and New Zealand coastwatcher organizations’ behind-the-lines spies. Unlike the Allied coastwatchers, the Japanese received little support from the Solomons tribesmen. Naval observers had very limited effect. Possibly their greatest role lay in tracking Allied air raids. Here they recorded the numbers and type of aircraft in attacks or over an area, plus the hour, altitude, and general characteristics of attacks. Much like radio-traffic analysis, the macro data could reveal things about Allied strategy.
Japanese Navy aircraft did the scouting and aerial photography. The Japanese were behind there, too, only beginning to procure specialized cameras in 1941, with only a handful obtained both then and in 1942, and a somewhat improved model just starting to appear in 1943. Half a dozen of the trainees of the first (small) class of photo interpreters were sent directly to Rabaul late in 1942. And no specialized photo-reconnaissance squadrons existed in the Japanese Naval Air Force throughout the period of the Solomons campaign. Given standard reconnaissance altitudes of nearly 33,000 feet, the Japanese cameras generated pictures with a ground resolution of about 1 : 20,000 (as against a U.S. standard for tactical planning of 1 : 5,000), and the handful of imagery interpreters were challenged by the volume of the photography.
As for the special-duty group, the 8th Communications Unit supported the Eighth Fleet and the Solomons area command, called the Southeast Area Fleet. Commander Hideo Ozawa, the unit’s executive officer, supervised the special-duty people, with Lieutenant Kenichi Ogimoto, a veteran of special duty at the Japanese embassy in Washington before the war, as direct boss. Their radio-monitoring complex, located in a palm grove off the west end of the runway at Vunakanau air base, had a barracks, a receiving center, and two radio direction-finding huts. The Japanese repeatedly built up the unit. A draft of 60 additional radiomen was added late in 1942. By the end of the campaign the special-duty group would operate 22 receivers, 4 monitors, and 6 RDFs.
Intelligence from all sources flowed into Southeast Area Fleet headquarters, and particulars would be forwarded to the Naval General Staff. The Japanese could be as bureaucratic as anyone. When, during the Guadalcanal evacuation, a special duty officer went on the air to mimic an Allied searchplane and feed false information to the Americans, he was reprimanded for violating naval communications regulations. The situation improved somewhat after March 1943, when Commander Naosada Arisawa joined the area fleet staff. Arisawa was a former special duty officer with the 1st Combined and understood what the radiomen were trying to accomplish. Near the end of the campaign Arisawa was reassigned to the convoy escort fleet.
Low Regard for Intelligence in Japan
For all their apparatus and accomplishment, the Imperial Navy never held intelligence in high regard. The U.S. Navy had the identical problem, but for it the triumph of Midway stilled objections to intelligence, the daily reporting assured commanders of its utility, and the Yamamoto shoot-down quenched any residual temptation to question the value of intelligence efforts. As a practical matter the Allies came to regard intelligence as a mechanism to identify targets that could then be eliminated by operating forces. The Japanese seemed to view intelligence more circumspectly—as a means of showing the dimension of the obstacles that must be overcome by the true samurai.
Though the Imperial Navy developed the same intelligence techniques as its adversaries, its sense of the limited utility of such information inclined the Japanese against devoting an effort equivalent to that of the Allies, and their intelligence never evolved into the same kind of supple instrument wielded against them. Relatively poor intelligence, compared to the Allied juggernaut, became one more factor in sealing the fate of the Japanese Empire in the Solomons.