Missing Clues and Cracking Codes in the Pacific War

By Elliot Carlson

Conflicting Views

Station Cast, Hypo’s sister unit hidden away in the Malinta Tunnel on Corregidor, the Philippines, didn’t think so. Early on 27 November, Cast’s analysts responded to Rochefort’s memo with an estimate of their own. Though similar to Rochefort’s overall picture, Cast’s memo differed in one important respect: It questioned Hypo’s finding that carriers and a large submarine force had arrived in the Marshalls. Cast was emphatic: “Our best indications are that all known First and Second Fleet carriers still in Sasebo-Kure area.” Cast closed its assessment with a kicker: “Evaluation is considered reliable.” 1 Cast copied Kimmel.

The admiral couldn’t ignore the Cast estimate. Kimmel had learned through the Navy grapevine that OP-20-G, the Navy’s main cryptographic organization in Washington, had circulated a disturbing memo stating that Cast’s findings should be given greater weight than Hypo’s, because radio reception was better in the Philippines and Cast supposedly was further along in breaking the IJN’s main operational code. 2

Rochefort was infuriated when he learned of this, and he vigorously disputed it. Kimmel also was agitated. The last thing he needed was squabbling among the Navy’s codebreakers. Now he wanted to know which assessment to believe: Hypo’s or Cast’s. The answer could bear on how he configured his forces at Pearl.

War Warning

Kimmel’s need for clarity assumed greater urgency in light of a new dispatch from Washington, this one received around midday on 27 November. It was worded a little differently from others, opening with a short, punchy sentence: “This is a war warning.”

The message alerted Kimmel that negotiations with Japan aimed at averting war “have ceased,” and added that “an aggressive move by the Japanese is expected within the next few days.” Washington warned of “an amphibious expedition against either the Philippines Thai or Kra Peninsula or possibly Borneo.” The dispatch told him to “execute an appropriate defensive deployment” as assigned in the Navy war plan known as Rainbow Five.

Then Kimmel got another message, this one sent by messenger from Lieutenant General Walter C. Short, the Army’s commanding general in Hawaii. The note paraphrased a warning the Army had just conveyed to Short. It paralleled the Navy’s alert to Kimmel, but it contained one different element: It directed Short “to undertake such reconnaissance and other measures you deem necessary.”

The reconnaissance order had gone to Short, but it related to Kimmel. The two had agreed earlier that while the Army was responsible for defending Pearl Harbor, the Navy would undertake any long-range air reconnaissance considered necessary. Kimmel now wondered: Should he put in the air some of the Navy’s approximately 50 PBY patrol aircraft on Oahu?

Not one of his aides recommended it. The improbability of a Japanese attack on Pearl Harbor was the accepted view among high-ranking officers there. It was certainly Kimmel’s opinion. As he said later, he thought the expression “war warning” meant simply that Japan “was going to attack some place,” but not Pearl Harbor. 3 Still, there were things he wanted to know. Where were the Imperial Navy’s carriers? And what about those subs and flattops Rochefort claimed were assembling in the Marshalls? Were they there? Did they pose a risk to Hawaii?

The questions swirling in Kimmel’s mind pointed toward Rochefort. Surprisingly, the two had never met personally to discuss the Japanese threat in the Pacific.

Spy Craft

Since 2 June 1941, when he had assumed leadership of Hypo, Rochefort had communicated with Kimmel through an intermediary, the Pacific Fleet’s intelligence officer, Lieutenant Commander Edwin T. Layton. Rochefort would have preferred a more direct relationship, but he didn’t complain. Layton was an old friend; the two had first met in 1929 on their way to Tokyo to learn Japanese. Eight years after leaving Japan, both remained fluent in the language.

Capable as the officers were, Rochefort possessed one skill that Layton lacked: cryptanalysis. He had been trained in this daunting and esoteric craft while on duty in Washington during the mid-1920s. Now, as officer in charge of Hypo, Rochefort managed approximately 100 officers and enlisted men—a zesty mixture of codebreakers, linguists, radio-intercept operators, and yeomen—based in different operations around the Island of Oahu. Hypo’s main listening post was 30 miles northeast of Pearl near a small coastal town called Heeia. The site wasn’t as favorable for reception as was Corregidor’s, but it managed to intercept messages across the entire Japanese communications system.

Radio operators collected messages encrypted in all kinds of codes. But they concentrated on those conveyed in two very secret, maddeningly complex systems, neither of which had yet been broken. Thus both Cast and Hypo relied not on codebreaking for their estimates of IJN ship movements, but on traffic analysis (TA). TA specialists inferred impending Japanese naval activity from patterns of IJN communication.

Instead of addressing the contents of a message as a cryptanalyst would, TAs tackled its heading, or preamble. From knowledge of call signs and the relationships between ships, they could often track IJN vessels to and from their ports. But TA had a downside: it required analysts to infer—in effect, guess—what was going to happen by drawing on the past. The Japanese would have to do in the future what they had done before.

Yet it was through traffic analysis that Rochefort had “spotted” Imperial Navy forces coalescing in the South China Sea and even showing up in the Marshalls. Drawing on transmissions picked up at Heeia and transported to Pearl by jeep, Hypo’s TAs prepared daily summaries describing IJN activity during the previous 24 hours. A liaison officer delivered the document by car to Layton at Fleet headquarters, about a mile away. Hypo’s summaries usually served as the basis for Layton’s daily 0815 briefing of Kimmel.

But with Japanese forces seemingly on the move, Kimmel now wanted to hear directly from Hypo’s officer in charge. He arranged to meet with Rochefort in Hypo’s space, a large, clammy room in the basement of the Navy Administration Building. Late in the afternoon of 27 November, Kimmel and an aide made their way down a narrow staircase into “the dungeon,” as Rochefort’s analysts satirically called their work site.

The Admiral and the Cryptanalyst

Kimmel and Rochefort were a study in contrasts. Stern, humorless, a taskmaster demanding in devotion to duty, Kimmel had been driving himself as hard as he drove others. Rochefort also led by example, but he did so by creating a different environment. The atmosphere in the basement was relaxed. Rochefort showed the way. In the dungeon’s damp and chilly confines, he usually wore a maroon smoking jacket and shuffled around in a pair of old slippers.

But not on 27 November. Everybody was all business this day. Grim, imposing in manner, Kimmel squeezed into the cramped space around Rochefort’s desk. Rochefort took over, holding court in dress khakis and puffing on his pipe.

He knew what was on his visitors’ minds: the conflicting carrier and sub reports and the war warning. “The commander in chief wished to know the basis for [Hypo’s] estimate, why we placed certain units in certain areas, wished to know the distinction or difference between the estimates of [Hypo] and [Cast], and what our opinion was regarding the location and direction of movement of the various Japanese forces,” Rochefort testified in 1944. “We discussed the matter at great length, at least an hour and a half, I would say.” 4

No one ever reported precisely what was said during the 90-minute meeting. But Rochefort’s thinking as of 27 November did become a matter of public record. Thanks to his testimony before seven panels convened during and after the war to probe the Pearl Harbor disaster, his responses to Kimmel’s concerns can be reconstructed.

Rochefort had no trouble answering Kimmel’s first question: What was going on in the South China Sea? If there was one thing Stations Cast and Hypo agreed upon, it was that the IJN was on the move there, steaming toward waters from where it could launch operations against Hong Kong, Malaya, Singapore, Borneo, and the Dutch East Indies.

But what about those subs and carriers in the Marshalls? Rejecting Cast’s challenge, Rochefort said he had no doubt that the submarines were there. Hypo’s traffic analysts had tracked them for days moving eastward toward the Marshalls. He conceded the carrier estimate was less certain, but from all indications there could be two small flattops in those islands.

With or without carriers, the Japanese forces building up in the Marshalls didn’t pose a threat to Hawaii, Rochefort told Kimmel. The naval force there wasn’t “the type of organization you would have for a striking force,” he said. He didn’t suggest that the Japanese forces in the Marshalls were harmless: “It was our assumption at the time that that group of submarines there, with or without the carrier group, would be used to secure the flanks against any possible move on the part of the United States,” Rochefort said later. 5

Rochefort turned out to be only partly right: IJN Fleet submarines had indeed arrived in the Marshalls, but their mission wasn’t defensive. By the time Rochefort briefed Kimmel, the large subs “sighted” earlier (there were 23) had departed the Marshalls and were moving silently eastward, toward Hawaii. Their orders were to take up positions around Pearl Harbor at choke points—shipping lines into and out of the harbor.

Neither Rochefort nor Kimmel had any way of knowing those subs had slipped away undetected. Because of Rochefort’s assessment, the Marshalls seemed less of a problem. Even the supposed carriers there were too small and too few to worry about (it developed there were actually none in the Marshalls). What Kimmel worried about now was the whereabouts of the IJN’s six heavy carriers: the Kaga , Akagi , Hiryu , Soryu , Zuikaku , and Shokaku . They had been quiet for weeks. Where were they?

‘The Japanese Won’t Attack Us’

The traffic analysts of Cast and Hypo in this instance drew similar inferences. On the same day Rochefort met with Kimmel, Station Cast conjectured that four of those carriers were in the Sasebo-Kure area, that is, the Inland Sea and the waters around Kyushu, Japan’s southernmost province. Hypo concurred. Rochefort passed on to Kimmel the evaluation reached that day by his own traffic analysts: “Carriers are still in home waters.” 6

Rochefort wasn’t overly alarmed about the carriers on 27 November. “The inability to locate more battleships and carriers was not considered in itself, as a bad sign,” he testified later. 7 He referred to a January 1942 memo by Layton showing that the location of IJN carriers had been “uncertain” as many as 84 days during the previous six months. 8 They usually turned out to have been in port or in Empire waters.

What Kimmel really wanted to ask, of course, was a question Rochefort couldn’t definitively answer: Were the Japanese going to strike any U.S. possession? Many high-ranking officers at Pearl doubted that. Vice Admiral William S. Pye, commander of the Pacific Fleet’s Battle Force, told Layton on 6 December: “The Japanese won’t attack us. We’re too strong and powerful.” 9 Others thought they wouldn’t strike because they couldn’t: Their planes were second-rate, their flyers had poor eyesight and were unskilled, the Japanese were incapable.

Rochefort had his own answer. He rejected stereotypes painting the Japanese as inferior. He had studied the Japanese navy and thought he understood it. He believed the IJN’s sense of its own limitations would act as a restraining influence on Japan’s strategic ambitions in the Pacific. He concluded that whatever Japan did in the future, it would act strictly in its own self-interest. “If we become involved in a war with Japan, the Japanese cannot possibly win,” Rochefort said later, reconstructing his thinking at the time. “Therefore, the Japanese will not proceed against the United States directly but will rather reach their goals by, say, [taking] Singapore, certainly Southeast Asia, maybe some of the islands but not the Philippines because this would bring them to war [with the United States].” 10 In short, the Japanese would be rational.

Unsettling Reassurances

Washington’s 27 November war warning rattled the foundations of Rochefort’s thinking: It expressly cited the Philippines as a potential Japanese target. This was something Rochefort had a hard time conceiving, but the warning was based in part on his own message of 26 November describing the massing of IJN forces in the South China Sea. Could those forces seize large portions of Southeast Asia and leave the Philippines unmolested? Rochefort now had to concede that if that U.S. possession were hit, the United States would be at war, and soon.

But the Philippines were distant: some 5,000 miles away. As Rochefort told Kimmel, Japan’s “objectives insofar as [Hypo] was concerned, did not include areas to the eastward of the [Marshall] islands,” that is, Wake, Midway, and the Hawaiian Islands. Hawaii wouldn’t seem to have anything to fear. “No one thought in terms of Pearl Harbor at the time,” Rochefort said later. 11

Ninety minutes after beginning his conversation with Rochefort, Kimmel could plausibly have departed Hypo’s basement feeling anxious. Four, possibly as many as six heavy carriers were unaccounted for. Most were believed to be in home waters, but they could be anywhere. The carriers were silent now. Maybe they were being held in reserve as IJN forces prepared to enter the South China Sea. There was no way to be sure.

But instead he might have come away relaxed. Rochefort’s appraisal of the Marshalls was reassuring. There was no attack coming from that quarter. Moreover, if Cast was right, there were no new forces in those islands. Even if Rochefort’s estimate was the correct one, the forces there were defensive.

Was Kimmel more impressed with a view stressing the unpredictability of the threat, or did he give more weight to the reassuring scenario in which everything that was going to happen would be 5,000 to 7,000 miles away? No one will ever know, but Rochefort’s briefing might have unintentionally reinforced the Fleet commander’s attachment to the conventional outlook at Pearl: Japan would strike—on the other side of the Pacific.

Rochefort and Kimmel did not meet again. Hypo’s traffic analysts continued to supply Kimmel, through Layton, daily summaries emphasizing the uncertainties of the situation. Day after day the carriers were reported silent. When on 2 December Layton informed the Fleet commander he couldn’t say with confidence where the IJN’s carriers might be, Kimmel exploded. “Do you mean to say they could be rounding Diamond Head and you wouldn’t know it?” he asked his intel officer. 12 Layton admitted he had no good answer.

Minimal Defenses

Prophetic as Kimmel’s now-famous question turned out to be, it raised no warning flag at Pearl. The defensive measures he instituted were minimal. He increased in-shore patrols and issued orders to the Pacific Fleet to depth-charge all submarine contacts in the Oahu operating area. But he decided against moving the Fleet out to sea, and he ordered no long-range air reconnaissance.

Rochefort’s 27 November briefing of Kimmel trailed events. Two days before that meeting, the 33 ships of the IJN’s First Air Fleet, commanded by Vice Admiral Chuichi Nagumo, slipped out of Hitokappu Bay and steamed into the north Pacific. Following the orders of Admiral Isoroku Yamamoto, commanding the Combined Fleet, Nagumo proceeded under strict radio silence on the first leg of a 3,500-mile voyage that would culminate 230 miles north of Pearl Harbor.

Ten days after meeting with Rochefort, Kimmel stood on the front yard of his home on Makalapa Heights, a hill behind the Navy Yard providing a perfect view of the harbor. He watched in disbelief as planes with rising suns on their wings destroyed his fleet. Ten days later Kimmel was relieved of his command. Little more than a month after that, on 23 January, the commission that President Franklin Roosevelt had convened to investigate the Pearl Harbor disaster issued its findings. The panel held Kimmel and his Army counterpart, General Short, “in dereliction of duty” for not conferring with each other “respecting the meaning and intent of the warnings,” and for failing to implement “appropriate means of defense required by the imminence of hostilities.”

Mistakes Were Made

Kimmel never accepted the panel’s findings. He agreed some mistakes were made in Hawaii but maintained that his bosses had withheld the contents of crucial diplomatic messages decrypted in Washington and not shared with the Navy’s commanders in Hawaii. Such information, he said, would have led him to mount a more vigorous defense. He never singled out Rochefort or any of his Pacific Fleet staff as responsible for the calamity.

Could Kimmel have reasonably faulted Rochefort for pointing him in the wrong direction? Clearly Rochefort made mistakes. When he conjectured that the submarines detected in the Marshall Islands were strictly defensive, he violated one of the cardinal rules of intelligence: Never assume. His judgment about those subs exceeded the evidence. Traffic analysis could locate the subs, but it couldn’t confirm what they were going to do.

Rochefort also erred by fixating on a single target: Southeast Asia. No question the Japanese were going to strike there, but by focusing single-mindedly on that region, he blinded himself to other possibilities. Rochefort said later that the Japanese had arrayed so many of their naval assets in the South China Sea, he didn’t think they had enough left for any other operation—another flawed assumption.

Along with many others at Pearl, Rochefort made his share of miscues during the days leading to 7 December. But it was Kimmel who made the decisions, and he must bear primary responsibility for those calls. Rochefort did put before him clues suggesting a more perilous situation. Kimmel didn’t heed those, nor did Rochefort fully grasp their meaning. The fact remains that in his 27 November briefing of Kimmel, Rochefort didn’t go against the grain of conventional thinking at Pearl. Given his credibility as Hypo’s lead cryptanalyst, he may have inadvertently strengthened the opinion Kimmel already held.

Rochefort was as shocked by Nagumo’s raid as everybody else. It contradicted everything he had come to expect from the Japanese and the Imperial Navy. Instead of being rational as Americans defined that term, they did something foolish. When a superior officer called the basement during a lull in the attack, he heard Rochefort’s bitter prophecy: “This is the end of the Japanese Empire as we know it.” 13

Rochefort never forgave himself for what he regarded as horrible lapses of judgment on his part. After the raid, he threw himself even more fervently into his work, not knowing that very soon, he and his Station Hypo colleagues would have a chance to redeem themselves.

1. Pearl Harbor Hearings (PHH), Part 6, pp. 2814–15.

2. Memorandum from Laurance Safford to ADM Harold Stark, cited in Safford’s memo to John F. Sonnett, counsel to Hewitt Inquiry, 17 May 1945, PHH, Part 18, Exhibit 151, pp. 3335–36.

3. PHH, Part 7, pp. 3026–27.

4. Testimony of Joseph J. Rochefort, PHH, Part 26, p. 216.

5. Rochefort testimony, PHH, Part 28, p. 871.

6. Communication Intelligence Summary, RG 80, Box 41, National Archives Records Administration II (NARAII), College Park, MD.

7. Rochefort testimony, Part 26, p. 216.

8. Memorandum from Edwin T. Layton to Roberts Commission, RG 80, Box 41, NARAII.

9. Edwin T. Layton oral history, U.S. Naval Institute, pp. 74–75.

10. Rochefort oral history, U.S. Naval Institute, p. 169.

11. Gordon Prange interview with Joseph J. Rochefort, Prange Papers, University of Maryland, College Park, MD.

12. Testimony of Edwin T. Layton, PHH, Part 36, pp. 127-28.

13. Prange interview with Rochefort.

Mr. Carlson, author of Joe Rochefort’s War: The Odyssey of the Codebreaker Who Outwitted Yamamoto at Midway (Naval Institute Press, 2011), traces his interest in the Pacific War to his first job out of college, writing editorials for the Honolulu Advertiser . After winning a Congressional Fellowship that took him to Washington, D.C., he began a journalistic career that included writing for The Wall Street Journal and Newsweek ; freelancing from Belgrade, Yugoslavia; and editing AARP’s monthly newspaper.

Elliot Carlson is a longtime journalist who has worked for such newspapers and magazines as the Honolulu Advertiser and the Wall Street Journal. His biography of Joe Rochefort won the 2012 Samuel Eliot Morison award for naval literature and the 2011 Roosevelt history prize. He lives in Silver Spring, MD, with his wife Norma.

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