Controversial Victory: The “Tanker War” Against Japan, 1942–1944

1 Introduction

In 1947, the United States Strategic Bombing Survey (hereafter Survey), a comprehensive review of the economic destruction of Japan, was published. Officially, a study of the effectiveness of firepower on the Japanese during the Second World War, buried in its detailed analysis, was an interesting conclusion. Citing Japanese reliance on oil as “obvious,” the report questioned and criticized the Navy’s submarine campaign that ultimately brought the Japanese oil industry to its knees.^[1] Specifically, the Survey stated:

Had submarines concentrated more effectively in the areas where (oil) tankers were in predominant use after mid-1942, oil imports probably could have been reduced sooner and collapse of the fleet, air arm, merchant shipping and all other activities dependent upon fuel hastened…and the fuel shortage might have been acute at the end of 1943 rather than a year later.^[2]

In later years, the Strategic Bombing Survey was criticized – correctly – for exaggerating the role of air power in the final victory as a rationalization for the creation of a separate and modern Air Force. But while the agenda of the survey certainly calls into question its assessment of submarine power against the Japanese oil industry, in some ways, the survey was fundamentally correct; while tankers were always considered important targets for submarines, a coordinated campaign against them did not occur until early 1944. Why was this the case?

The answer to this question is complex. While the Survey correctly noted that the Japanese reliance on oil exports was well known prior to the war, knowledge of the particulars of the Japanese oil industry and the ships supporting it was sparse at the onset of World War 2. Moreover, the submarine force of the early war years was in poor shape to conduct such a campaign. Years of cautious doctrine, poor strategic planning, and faulty weapon systems all ensured that early submarine operations against an elusive enemy were disappointing. Japan’s supply of oil sailed relatively unhindered.

Ultimately this changed through not only the development of new and aggressive tactics by the submarine force but also the massive expansion of intelligence and analysis. Through innovative leadership and coordination between signal intelligence and submarine planners, the Pacific submarine fleet was able to execute a brutal campaign against enemy tankers that effectively deprived Japan of its oil imports by late 1944. The results of the “tanker war,” however, were far from pre-ordained. The link between signal intelligence and submarines, so essential in winning the war, was slow in occurring due to the extreme secrecy exercised by both services. Doctrine and early war priorities were difficult to change. Entirely new systems of analysis and cooperation had to be created, tested, and implemented under enormous pressure and risk. The fact that the link between signal intelligence and submarines was made – and executed to such devastating effect – was largely the result of men of vision in both services that took enormous risks to change culture, methods, and means of execution.

2 Empty Seas: Japan’s Tanker Fleet 1942

In early January 1942, the captain of the submarine Gudgeon slowly raised his periscope, and his crew at general quarters was ready for instant action. Sailing for Japan within hours of the attack on Pearl Harbor, the Gudgeon was one of the first US submarines to enter enemy waters and was operating under new orders. The surprise attack on Pearl Harbor had changed the Americans’ distaste for unrestricted submarine warfare; after years of training to attack warships, Gudgeon was ordered to attack all vessels sailing in the enemy’s zone of operation, especially merchant targets. The captain of Gudgeon almost certainly expected targets to be waiting in abundance; after all, Japan’s reliance on overseas imports for everything ranging from food to raw materials to feed its war machine was well known. But the seas were empty save for one target.^[3] Where was the merchant fleet?

Gudgeon’s experience was not unique to the submarine fleet patrolling enemy waters in 1942. The first months of the war were a harsh introduction to challenges of searching for an elusive enemy in the vastness of the Pacific. Targets were there in abundance – but finding them was an enormous challenge. The size and extent of Japanese economic operations were unknown to the Americans who had ignored many elements of Japan’s rapid industrialization – and its early conquests – that molded the development of Japan’s economic power at sea.

Sea power was tied closely to Japan’s rapid industrialization at the beginning of the twentieth century as it emerged from centuries of isolation. Industrialization obviously required oil, a commodity Japan lacked. Overseas imports were the answer, but Japan had a very small merchant fleet capable of transporting oil. Contracted foreign hulls, a process that was common in the international oil trade, served to provide Japan’s needs – so long as it was at peace.^[4] Peace, however, proved elusive and war brought isolation. An Allied partner in World War One, Japan, felt cheated and insulted by the West in the aftermath of Versailles and turned to a deliberate policy of limiting reliance on its former allies. While Japan still had to go overseas for its raw materials, it could – and did – control other parts of the process. Shipbuilding became a priority, especially the creation of an independent merchant marine. By 1941, this merchant fleet consisted of over 6,000,000 tons of combined shipping.^[5]

From the outset, however, the system of building and controlling merchant ships was convoluted between commercial and military interests. While the Japanese government was controlled by the military, commercial shipping was built and manned by civilian corporations. As Japan moved toward war, there was an understanding that ships would be “drafted” for military use as circumstances required.^[6] In general, tankers avoided this “draft” as oil provided to commercial industry directly aided the war effort. But the fleet was stretched thin; planning for war with America had completely underestimated shipping requirements, especially the need for oil tankers. Japan possessed 500,000 tons – 49 hulls – of these vital ships that were now stretched across a new and expanding Empire, a very small number.^[7] To meet wartime industrial as well as operational needs, tanker construction was given priority in shipyards from 55,000 tons in 1942 to 930,000 tons over the next 2 years – not including cargo ships that were converted to carry oil.^[8] These challenges and weaknesses were unknown to Allied intelligence.

Also unknown to Allied intelligence were the sources of Japanese oil and the routes by which it was being shipped to Japan at the onset of the war. This was far more complex than would appear. Japan had launched its war in a quest for resources, striking south to the oil fields of Borneo and the Dutch East Indies.^[9] Japan’s conquests for oil resources would ultimately include the Philippines, Hong Kong, Singapore, and Sumatra where oil could not only be collected but also be refined. The diversity of these sources presented a unique problem for Americans intent on targeting them. Unlike the large and established oil shipping ports of America and Europe, Japanese oil sources were spread out and, worse, had multiple small ports where oil was routinely shipped – often seemingly at random. Early intelligence had a very difficult time pinpointing where these ports were and, more importantly, the level of activity at each. As we shall see, the chaotic nature of the management of the Japanese supply system made effective analysis even more challenging.

The size of the operational area compounded this problem. Simply put, the Pacific was huge, and Japan’s initial conquests were spread over an incredibly broad area. Submarines simply could not patrol such a vast area in an attempt to pinpoint specific economic operations. Nor, it seemed from initial operations, were US submarines completely up to the task of waging an economic war.

3 Submarines: Doctrine and Practice

In the war’s aftermath, author Theodore Roscoe described the first 6 months of the submarine war as a robust economic campaign with US submarines aggressively seeking out and sinking merchant targets, sending over 40 to the bottom.^[10] Written immediately in the aftermath of the victory in the Pacific, the story described a well-planned and executed campaign that ultimately brought the enemy to its knees. It was a common myth. The reality was far different.

Early US submarine operations against economic targets were at best inconclusive and worst very poor. At a time when German U-boats in the Atlantic had sunk over 200 ships (and 22% of the Allied tanker fleet), the US submarine force’s efficacy against Japan was at best minuscule. Moreover, many of the “sinkings” in the postwar analysis could not be verified as submarine captains simply assumed destruction at the sound of an explosion which, determined later, were often the result of faulty torpedoes.^[11] There are a number of important reasons for this poor performance, primarily focused on prewar doctrine and training.

As a warfighting machine, the main US submarine – the GATO class – was a superior weapon for economic warfare.^[12] But it was never intended to fulfill this role. The GATO had been designed to meet the requirements of War Plan Orange, a Mahanian-style battle plan conceived in the interwar years as a means to bring the Japanese Navy to a decisive battle. Orange envisioned that upon hostilities the surface Navy – primarily battleships at the time – would race forward to engage the enemy fleet. Submarines were intended as scouts, sailing ahead of the main battle force. The GATO – rugged, fast, capable of sailing long ranges without refueling and with deep diving capability – was designed for this role. Submarine captains of the class were deeply ingrained in the doctrine of stealth and caution – precisely the opposite of what was needed in an economic war. Wartime reality soon demonstrated that the GATO-class submarine was a poor scout; a very low height of eye and limited range sensors made this readily apparent. Cautious tactics such as firing torpedoes from deep depth or at long range simply didn’t work. And there were no tactics or techniques at all for targeting merchant power.

Wartime experience would sharply reduce these elements as doctrine changed and submarine captains became more aggressive with their boats. But while experience and the harsh lessons of war would improve submarine performance tactically, the problem of finding targets remained a mystery.

4 Intelligence

The answer to penetrating the mystery of Japan was obviously intelligent, but this was simply not available in 1942. Knowledge of Japan among conventional military officers was limited to occasional “military to military” contact or from those who had served in rare overseas postings.^[13] Unknown to all but a few senior officers, there was intelligence to be had, but it was from a technical and highly classified source and as such was not available to the submarine fleet. This was in the realm of signal intelligence.

Signal intelligence was not new to the military, but it was both highly secretive and very selective. First begun in 1917 by the US Army and Navy, during peacetime signal intelligence operations had continued under the infamous “Black Chamber,” a Department of State-sponsored signal intelligence unit specializing in diplomatic codes. But code-breaking had severe limitations, especially after a number of highly publicized scandals made Americans aware of the extent of their government’s activities.^[14] In the interwar years the Army and Navy proceeded cautiously, maintaining small, highly secret staffs that continued to target Japanese codes under the moniker of “training” so as to not to violate legal statutes. This “training” had an impact. Prior to Pearl Harbor, the Army’s Signal Intelligence Service was reading most of Japan’s diplomatic code (“Purple”) while the Navy had made significant penetrations into the mainline Japanese Navy code (JN-25).^[15]

But could signal intelligence be used for an economic campaign? At the onset of the war, Navy code-breaking seemed to hold the most potential. The Navy had spectacular success with its signal intelligence early in the conflict. As is well known to history, the code-breaking unit at Pearl Harbor (“Hypo,” later “FruPac”) under the brilliant CDR Joe Rochefort made enough inroads to the mainline Japanese naval code (JN-25) to allow for Navy intercept in the Coral Sea, and decisive engagement at Midway. While the mainline codes were a priority, they were not FruPac’s only effort. The codebreakers also intercepted simpler codes detailing other types of operations, including codes that gave hints for the Japanese routing of individual warships and merchant vessels. This caught the attention of Lieutenant William “Jasper” Holmes, an individual who would have an almost incalculable impact on the future economic war.

Holmes himself was an interesting figure. A career naval officer (and former submariner), Holmes had been medically retired from sea duty, normally an act that would end a career.^[16] Rochefort, desperately in need of administrative assistance, hired Holmes to work at Hypo in an attempt to provide some sense of organization to the (at best) eclectic codebreakers. In the course of his duties, Holmes had access to the most basic of intercepts and put them to use. As an academic exercise, Holmes began plotting merchant traffic on a chart of the Pacific, using available intercepts that were easily read.^[17] Although Rochefort was originally skeptical of Holmes’ activities, he did give him enough access to signal intercepts for his chart, which rapidly illustrated its value. Holmes demonstrated that through rudimentary analysis it was possible to track not only merchant vessels but also, as more information became available, some smaller warships as well.

As a submariner, Holmes was well aware that the information on the FruPac chart offered tremendous potential to the submarine fleet now that merchant vessels were legitimate targets. There were, however, a host of problems. Information on merchant vessels was still very limited – type of ship and its cargo could not yet be easily correlated. But the larger issue was not the gathering and collation of material, but rather disseminating it. Sharing information with operational units violated established security procedures for dissemination of signal intelligence, which was only to be done for the highest priority targets.^[18] Without such a target, the risk of compromise was considered too high. And, of course, there was some question as to whether submarine intercept would actually work. Without meeting the stringent security conditions and conducting an actual test, Holmes’ chart remained just an academic exercise.

In February 1942, a target appeared that met the targeting criteria. The Japanese port director at Truk routinely transmitted the arrival and departure of military units in a minor code that FruPac had been reading for some time.^[19] A message was intercepted reporting the pending arrival of a Japanese aircraft carrier at Truk. Such a target seemed to satisfy the dissemination criteria. Holmes asked Rochefort’s permission to pass the information to Commander Charles W. Styer, then ComSubPac’s Chief of Staff and a friend from Holmes’ days in submarines.^[20] Rochefort agreed, providing the information was “sanitized” to protect the source. The submarine Grayling was dispatched for intercept, arriving off Truk to spot the enemy carrier departing and sailing over the horizon. Although no attack occurred, the fact that the carrier was precisely where FruPac said it would be demonstrated to ComSubPac that a new targeting source was potentially available. Holmes’ experiment and his informal relationship with ComSubPac was the first real tie between submarines and signal intelligence.

But the tie was tenuous. Real information was scarce and tended to be relayed after the fact. There was still a great deal of guesswork as to the enemy’s intentions, and submarines were often not in a position to take advantage of a sudden operational break. And the security issue remained. Until more information could be provided to guide submarines effectively to economic targets, tankers sailed at will.

5 Expanded Analysis: Enter the Army

In late December 1941, Wall Street lawyer Alfred McCormack was about to have the most important meeting of his life which, unbeknownst at the time, would have an enormous impact on the Pacific War. Secretary of War Henry Stimson was about to brief him on the war’s best-kept secret and ask him to play a vital role.

As a senior government official, Secretary of War Henry Stimson was well aware of US signal operations in the interwar period including the “Purple” diplomatic breaks that warned of rising Japanese tensions prior to Pearl Harbor.^[21] Fully briefed on initial wartime operations, he saw the potential of signal intelligence – as well as the weaknesses.^[22] Initial lessons from the Pearl Harbor attack indicated the need for more personnel, more analysis, and above all organization. Stimson concluded correctly that the coordination of intelligence required a highly organized and disciplined leader. He subsequently summoned an old friend who possessed exceptional organizational skills used to working in a diverse environment.^[23] In Stimson’s mind, McCormack had recognized exceptional organizational skills at a senior level, precisely what wartime signal intelligence required.^[24]

McCormack emerged from the meeting a new Colonel, with direct tasking from the Secretary to review signal intelligence operations and recommend improvement. He first examined processes, specifically methods of analysis of available signal intelligence. Methods and processes were random. Often a signal section chief literally had to “carry the information around in his head;” were he to make any connection between intercepted messages, it was largely a matter of memory and individual skill.^[25] The intercept process itself was locked in conflicting priorities and classifications among the different groups handling the material.^[26] No cross-checking or examination of related material was conducted to confirm the information in the intercepts or generate a strategic picture. McCormack concluded that a new process was required and ultimately a new organization.

This led to the formation of the Military Intelligence Service (MIS) to coordinate signal activities between agencies in the War Department, and in the spring of 1942, Special Branch. The ultimate objective was to provide strategic and operational intelligence to forces in the field based on long-term analysis of signal intelligence. This included establishing greater coordination between Army and Navy with the goal of devising a base picture of Japanese capabilities and intentions, specifically to coordinate intercepts in the Far East Section dealing with enemy long-term operations and economic conditions.^[27] Information was obtained using code-breaking systems already in place and in use by both Services, but it was examined with a different eye. Rather than examining intercepted messages for immediate content that could be used operationally, Special Branch conducted a close and continuous study of an evolving intelligence problem to determine the connectivity of events.^[28]

MIS’s Army analysts were not idle in their efforts to derive a picture of the Japanese strategic and economic condition; by late 1942, the ongoing effort to derive a strategic picture of Japan began to yield results. Pieces emerged slowly. Through exhaustive analysis of the available material and inter-service research, MIS reported a “pretty clear” picture of the Japanese movement of supplies by sea to and from Indochina and Thailand, the first-time strategic economic analysis of enemy operations. Problems that the Japanese were experiencing in their supply operations slowly became apparent, identifying weaknesses for potential exploitation.^[29] Unfortunately, strategic information on shipping was difficult to obtain; there was no single source or code that provided an entire picture, only snippets of information. This challenge would put McCormack’s principles of organization and analysis to the test. In his own words:

[…] using garbled texts, incomprehensible ship names, abbreviations and alphabetical designators, incomprehensible ship tonnage figures, loading reports and ship schedules; from this a base intelligence picture of the Japanese shipping operation was devised.^[30]

This picture had an impact. Prior to the MIS study of the Japanese merchant marine, Navy estimates of merchant tonnage employed in the South Pacific were 3,000,000 tons; logically, if the Japanese merchant fleet were this small, the secondary targeting of merchant power was sufficient, the priority assigned to warships for submarines a sound strategy. The MIS study indicated that at least 4,500,000 tons were being used by the Japanese – ultimately, this was determined to be much higher.^[31] This made it clear to naval planners that more effort would have to be directed against merchant power if the war was to be won.

Practically, MIS began to tabulate their data on the Japanese merchant marine into a working operational document known as the Shipping List.^[32] The list contained a wealth of tactical information, including lists of known Japanese merchantmen, ship tonnage, likely points of transit, and estimate of use. This was sent to Holmes at FruPac, where analysis of the shipping list combined with FruPac intelligence began to provide a rough picture of high operational areas that submarines could target.^[33] In his own postwar narrative, Holmes noted the reciprocal relationship with MIS that began with the sharing of the Shipping List, as FruPac now began making requests of MIS for specific background research and additional analysis to improve its own operations.^[34]

The Shipping List also began to provide indications on enemy port activity, long a mystery for the submarines.^[35] While the placement of submarines in natural “choke points” and off larger ports had yielded some results,^[36] very little was known of supply loading or unloading areas that dotted the Chinese mainland and the outer fringes of the Empire. Through painstaking analysis, MIS compiled information on Japanese shipping activity from these ports which could be used to direct submarines to these areas.^[37] This had mixed results as the Japanese often shifted loading operations, seemingly at random.^[38] For the analysts, this frustrated efforts which were long and often unproductive. But it was an important beginning in the long road to get a strategic picture of enemy operations.

These were important steps toward truly attacking economic power. But more information was needed if vital economic assets – especially the oil tankers – were to be effectively targeted. It would soon come through two decisive code breakthroughs.

6 The Codes

In late 1942, a code break would be made that would have a significant impact in raising the curtain further on Japanese economic operations and subsequently lead to increased effectiveness by the US submarine force. It was, interestingly, based on the best of intentions by the enemy.

By late 1942, it was becoming increasingly apparent to the Japanese that the original plan of a protective “bubble” around the Empire through which merchant power could sail at will had failed. US submarines, while performing marginally compared to their German counterparts, were still making attacks that were frustrating the Japanese. The clear answer was the development of a convoy system. But this presented a dilemma. The Japanese Navy, like many other navies, had been disdainful of convoying as “defensive.” The concept was barely studied and certainly not considered worthy of naval officers destined for greater authority. But something had to be done – or at least attempted.

The results were not what the Japanese hoped for. While the Allies had learned to rigidly control large convoys, the Japanese formed smaller convoys that were put together on an “ad hoc” basis involving a wide range of personnel, commands, and authorities in a very convoluted bureaucratic system of supply. Coordination among a wide range of commands of both Services (and civilians) required a means of communication that could be used at lower, operational levels – in other words, a relatively simple code. The Americans were listening.^[39]

The solution the Japanese devised for joint communication was a four-digit cipher that the Americans referred to as the “Maru” code.^[40] The code coordinated operations among the escort commander, port director, and convoy commander as well as other interested parties along the convoy route, such as garrison commanders. In theory, the system was fairly complex and should have guaranteed a relative degree of safety to the Marus. But there were weaknesses, most notably that elements of the code and its use were similar to civilian merchant codes used in peacetime, a fact noted by a Coast Guard Officer assigned to FruPac who was familiar with prewar merchant codes.^[41] For the economic war, it was a literal gold mine.

The wealth of information available through the Maru code tightened the relationship between FruPac and ComSubPac. Submarines could now be placed in a position to intercept multiple ships rather than being forced to chase individual targets. Although there was initially some hesitation from the codebreakers about providing convoy information to submarines for fear of compromise, in the case of the Maru code the potential benefit clearly outweighed the potential loss, especially as if the Japanese felt the Maru code had been broken (considered unlikely by the codebreakers), this information would not compromise the increased reading of other, more sophisticated codes used by the Japanese, considered by the Japanese High Command to be unbreakable.^[42] Maru information began to flow immediately. Through the relationship between Holmes and ComSubPac Operations officer Lieutenant Commander Richard Voge, daily briefings on new sightings as well as the position of submarines capable of response were begun between the commands.^[43] A telephone link was established between watch officers, allowing for 24-h coordination.^[44] Signal intelligence had become operationalized.

But while the Maru code and subsequent breakthroughs detailing convoy operations were significant, its impact did not immediately represent a titanic shift in submarine strategy. Maru information would take time to develop, as would an analysis of information to the point that it was of immediate tactical use. By mid-1943 efforts by MIS and FruPac to conduct strategic analysis were beginning to penetrate many of the critical areas of enemy economic operations but were still painstakingly slow. If oil tankers were to be effectively targeted, what was needed was a breakthrough that could tie the information together to create a practical planning tool. This breakthrough occurred in mid-1943 from an unusual source.

In the Japanese supply bureaucracy, the Army was responsible for the loading of all cargo – the logic being that the vast majority of cargo would in fact be used in Army theaters – while the Navy controlled the shipping (in cooperation with civilians) and escort forces. The Maru code described ships, but the cargo those ships carried was an Army responsibility – including communications. The Japanese Army mainline code (designated 2468) was complex and virtually unbreakable, but also very difficult to use by operators – especially those outside of the Army now involved in wartime supply. Once again, the Japanese responded to the increased demands of wartime operations by simplifying the code. This new code was easier to use, but also lacked the security of the original mainline system. In April 1943, codebreakers at SIS and MacArthur’s Central Bureau (CB) spotted the changes in the code and traced inconsistencies in past intercepts; analysts at CB and Arlington Hall subsequently performed complex comparisons on IBM sorters to determine connectivity in the codes.^[45] Sharing information, the two cryptanalytic organizations broke the code. It was to have an enormous impact on the economic war.

The break into the “Water Transport” code had special significance for the undersea campaign, a fact that was immediately apparent to the codebreakers. Initial messages broken and correlated contained a wealth of information concerning merchant shipping, much of which was previously unknown.^[46] These included individual orders and reports concerning ships that the Japanese army was using – or would be using – for future transport. The code spelled out in remarkable detail not only exact sailing orders for specific ships but also the composition and tonnage of priority cargos, information that had previously alluded planners. Analysis of this information provided a comprehensive picture of strategic materials including rice, pig iron, rail transport, and other items and systems vital to the war economy, all of which could subsequently be targeted by submarines, and later, aircraft.^[47] Strategically, this level of detail was precisely the information needed by ComSubPac to direct its submarines against high-value merchant targets and exploit enemy weaknesses. It was finally possible to target oil directly.

7 Unleashing Hell: The Tanker Offensive

Upon assuming the position of ComSubPac in April 1943, Vice Admiral Lockwood issued an order making Japanese tankers priority targets for submarines, reflecting his own strategy in targeting economic power.^[48] This order had not been particularly effective. Tankers were well escorted, and as vessels, they were difficult to sink as they were both large and compartmentalized.^[49] Moreover, as tankers were high-priority assets for the Japanese, many of the tankers that had been sunk were replaced as part of an aggressive shipbuilding campaign, so much so that overall tonnage available to the Japanese in mid-1943 remained roughly the same.^[50] But now, with combined information from the long-term strategic analysis, the shipping list, and Maru and water transport intercepts, it was possible to pinpoint target them for destruction.

With specific information and a solid campaign plan, the “tanker war” commenced in late 1943. Orders were issued to the submarine fleet directing that tankers were a top priority, second in importance only to aircraft carriers and battleships. It was a remarkable strategic shift. The results were immediate. By the end of 1943, submarines directed to patrol areas based on signal intelligence targeting oil sank 15 tankers, more than Japan had lost in the entire war up to that point.^[51] Amazingly, the Japanese did not seem to take this threat seriously. In February, Grand Escort Headquarters,^[52] organized two special convoys in the East China Sea to ship fuel to Japan composed of seven tankers (two and five, respectively); incredibly, only one escort was assigned to each. Intelligence was good and submarines were waiting; only one tanker survived.^[53]

The slaughter continued with devastating impact. Apart from the obvious impact of ship destruction and loss of cargo, the second- and third-order effects on the Japanese were incalculable. In the last half of 1943 – the start of the tanker war – oil imports were reduced from 1,500,000 barrels a month to 1,200,000; by June 1944, this had been cut in half. Of the 43 tankers lost in the first 6 months of 1944, 27 fell to submarines.^[54] In desperation, the Japanese High Command took the unheard-of step of reallocating steel destined for Navy use to construction of new tankers, but even with rare steel allocated to the Marus, losses simply could not be replaced. This program effectively stopped with the final cutting of the route to the Philippines in late 1944.^[55]

Militarily, the Japanese Army and Navy began to grind to a halt for lack of oil. As an illustration of how serious the shortage of oil had on operations, in late 1943 – the early months of the tanker war – Admiral Tokusaburo Ozawa’s carrier fleet returned from Rabaul to replace aircraft losses and train new pilots, but discovered that the oil shortage was so severe that he was obligated to sail to Singapore to be closer to the oil sources of Borneo and Sumatra.^[56] While placing the fleet closer to these oil sources provided the ships with fuel, it also put the task force in a strategically recessed position. Ozawa’s desperate effort to reach the Marianas from this distant area ended in catastrophic defeat at the battle of the Philippine Sea in June 1944. A major factor in this defeat lay in the poor training of Japanese pilots, undoubtedly due in part to the lack of fuel available for this purpose.

The campaign against tankers was an important element of the submarine war of attrition of 1944, a war that was becoming increasingly effective. Japanese attempts to use other merchant vessels to carry oil completely failed as the fleet was systematically destroyed. With the new intelligence, in a 12-month period over 603 merchant ships were sunk, totaling over 2.7 million tons – more than the total sunk in all previous years combined.^[57] By war’s end, the Japanese merchant marine simply ceased to exist, enemy sailors speaking of “walking on periscopes” as a means to deliver vital supplies to garrisons that were increasingly isolated. What remained of Japanese oil infrastructure was now left to the Army Air Corps to destroy as their Super Fortresses reached the home islands.

8 Conclusions

In the aftermath of the war, the “silent service” remained so about its wartime activities, the histories of the submarine war void of discussions of intelligence support, campaign planning, or the liaison with the codebreakers whose activities remained classified until the early 1980s. In this environment, the claims made by the new Air Force in the Strategic Bombing Survey could be debated, but not disproven without an examination of the full extent of the intelligence required to effectively target the Japanese tanker force.

Yet even after the revelations of signal intelligence were made, some historians argued that the submarine war could have been far more decisive if economic power were targeted immediately in the conflict.^[58] In hindsight, there is always a case for pointing out errors in war. But this argument fails to account for the years of training submarines had in conducting a conventional campaign, established doctrine, and above all the ability to find economic targets of impact such as oil tankers. Signal intelligence, in its infancy at best at the start of the war, developed and matured rapidly – but even then, its use against economic targets was a matter of controversy and debate over other wartime priorities and security concerns. The fact that the link between submarines and signal intelligence was made and evolved into the most devastating undersea campaign in history is a credit to those from both fields – operational and intelligence – who recognized the potential of each and were willing to act. It was only then that a campaign could be launched against Japanese oil tankers that proved decisive.