7

The Code Breakers

In addition to the men who drove the ships, flew the planes, or manned the guns—and those who some months later waded ashore carrying M-1 rifles—there were others whose contributions to victory in the Pacific were of an entirely different sort. Among the most consequential were those whose job it was to intercept, decrypt, and analyze Japanese radio traffic. In a windowless basement room at Pearl Harbor officially dubbed the Combat Intelligence Unit and which those working there called “the dungeon,” more than two dozen men toiled around the clock in an effort to glean useful intelligence out of the Japanese radio messages that were plucked out of the ether every day by the radio receiver at He’eia on Oahu’s north shore. It was the most secret organization in the U.S. Navy. Some of these men (called the “on-the-roof gang,” or “roofers” in the workplace vernacular) wore headsets and transcribed the blizzard of dots and dashes into number groups or Japanese kana characters.* Others sought to find patterns in those characters by running primitive IBM card-sorting machines that spewed out millions of punch cards each day. Still others sat at desks or tables and worked through tall stacks of intercepts, looking for repeated codes or phrases that might provide a hint about Japanese movements or intentions. It was an eclectic team of idiosyncratic individuals that collectively played one of the most important roles in the Pacific War, and particularly in the Battle of Midway.¹

As far back as World War I, the United States had been successful at breaking the Japanese diplomatic code. In the 1920s, a clandestine organization headed by Herbert Yardley and rather dramatically dubbed “the Black Chamber” devoted itself to breaking the diplomatic codes of several nations, including Japan. Their success had allowed the Americans to take a hard line at the 1921–22 Naval Arms Limitation Conference in Washington, where they had proposed that 10:10:6 ratio in battleship tonnage for the United States, Great Britain, and Japan. Though the Japanese were holding out for a 10:10:7 ratio, the American chief negotiator—the secretary of state and future Supreme Court chief justice Charles Evans Hughes—knew from reading intercepted Japanese secret messages that Tokyo would accept the 10:10:6 formula rather than let the talks fail.

Six years later, when the State Department was preparing for the London Naval Conference, Yardley sent President Hoover’s new secretary of state, Henry Stimson, a batch of decrypted messages that revealed Japan’s negotiating strategy. Instead of praising Yardley, Stimson was horrified. He was said to have remarked that “gentlemen do not read other gentlemen’s mail.” Whether or not he actually made this statement, he shut down the Black Chamber and ended, temporarily, efforts to read Japanese diplomatic messages. Yardley, who apparently could not resist claiming public credit, got even with Stimson a few years later by exposing the operation he had led in a series of magazine articles, and then by publishing a memoir, The American Black Chamber (1931), in which he revealed the once highly secret operation. For their part, the Japanese complained that the Americans had been cheating and resolved to improve their codes.²

The U.S. Navy was less fastidious than the State Department; efforts to break the Japanese Navy’s operational code continued uninterrupted. This quite separate effort began in 1924, when the communications intelligence organization was established on the top floor of the Navy Department building in Washington under the Code and Signals Section. Placed under the director of Naval Communications, this office was designated as OP-20-G. For almost twenty years, OP-20-G was the private fiefdom of the gifted and eccentric Commander Laurance F. Safford, a lugubrious, bespectacled 1916 Annapolis graduate with darting eyes and disheveled hair who looked, one coworker said, as if “he had been scratching his head in perplexity.”³ It was Safford who established the unit’s two satellite stations, one in Manila in 1932 called Station Cast, and Station Hypo in Hawaii in 1936.* It was also Safford who recruited the first team of analysts who became key players in the wartime code-breaking effort.**

One of those whom Safford recruited was Ensign Joseph J. Rochefort, who had enlisted in the Navy in the last days of World War I and earned a commission after graduating from Stevens Institute. Rochefort was a tall, thin, and soft-spoken man whose ready smile disguised a fierce intensity. He had not set out to be a code breaker, and never requested the duty. Nevertheless, in 1924 his former commanding officer on the fleet oiler Cuyama, Commander Chester Jersey, when asked to nominate someone for the Code and Signals Section, recalled that Ensign Rochefort had been particularly good at crossword puzzles. He sent in Rochefort’s name, and in 1925 Ensign Rochefort became Safford’s number two man. Four years later, the Navy sent Rochefort to Japan for a three-year tour, ostensibly as an attaché but really to study Japanese language and culture.⁴

 Joe Rochefort, seen here as a captain in a postwar photograph, was a key figure in the American code-breaking apparatus before and during the Battle of Midway. (U.S. Naval Institute)

Both Safford and Rochefort proved adept at the tedious and exacting work of code breaking, but they could not remain continuously in the job. Because the Navy expected its officers to serve at sea if they expected to be promoted, the two men adopted the practice of filling in for one another: Rochefort took over OP-20-G when Safford went to sea, and Safford resumed command when it was Rochefort’s turn to deploy. In June of 1941, with war looming, Safford sent Rochefort, by now a lieutenant commander, out to Hawaii to take over as the head of the Combat Intelligence Unit (CIU), colloquially known as Station Hypo. There, Rochefort had particular responsibility for breaking what was called the “Japanese admirals’ code.” Station Cast in Manila remained focused on trying to decrypt the Japanese Navy’s operational code.⁵

As it happened, the Japanese made little use of the admirals’ code, and for several months—including the critical month before Pearl Harbor—Rochefort and his team spent a lot of time chasing down blind alleys. Since they did not have access to the intelligence gathered from either the Japanese diplomatic code (known as “Purple”) or the operational codes, Rochefort could not share information gathered from those sources with his boss, Admiral Kimmel.⁶

Instead, Rochefort and his team relied heavily on traffic analysis, that is, an examination of the external character of the messages rather than of their content. The analysts noted the call sign, the message classification, its level of importance or precedence, the frequency of transmission, its length, and the location of the transmitter to draw conclusions about what the message might mean in terms of Japanese naval movements. If, for example, the volume of message traffic suddenly surged, it could mean that a fleet was getting under way. Of course, since fleets at sea often maintained radio silence, the absence of radio traffic might be equally significant. When messages to or from the Kidō Butai suddenly stopped, that could be as important as a sudden flurry of messages, or even more important. In addition, it was occasionally possible for a veteran operator at He’eia to determine the identity of the sender of a particular message by recognizing the characteristic tempo or cadence (called a “fist”) of his Morse-code transmissions. If the sender was known to work at a specific base or ship, it provided the Americans with one more piece of intelligence. Traffic analysis had limitations, however. On one occasion, when radio traffic showed that several destroyers usually associated with a particular Japanese carrier were in the Marshalls, Rochefort assumed the carrier was there, too. He was wrong. His rare error was an example of how the analysts had to apply intuition to determine the utility of the intercepts.⁷

None of this, of course, helped the Americans predict the attack on Pearl Harbor, and after December 7 it seemed to many that a housecleaning was in order. Just as Kimmel was shoved aside in Hawaii, Safford was replaced at OP-20-G by Captain John R. Redman. A member of the Naval Academy class of 1919 who had graduated early for service in World War I, Redman had excelled in athletics. A standout on the football and lacrosse teams, he was also captain of the wrestling team, a sport in which he competed at the 1920 Olympic Games in Antwerp, finishing fourth as a light heavyweight and just missing an Olympic medal. Redman had a strong personality and long service as a communications officer in cruisers and battleships, but no real experience or expertise in code breaking. In fact, his prime qualification for his new job may have been that his older brother, Rear Admiral Joseph R. Redman, was the director of Naval Communications.

 Captain John Redman, seen here as a rear admiral in a postwar photograph, headed up the intelligence office in Washington (OP-20-G) and was often suspicious of Rochefort’s analysis. (U.S. Naval Institute)

Captain Redman’s appointment not only shelved the experienced Safford, it introduced a new tension into the cryptanalytic community. Redman did not know Rochefort or any of the veteran cryptanalysts personally. Perhaps because Rochefort was Safford’s appointee (and not an Academy graduate), Redman was loath to take Hypo’s assessments at face value. Much later, Rochefort recalled, “As long as Safford was in Washington, I just about knew what to expect…. It worked very nicely on a personal basis. It was when other people became involved in it as part of the expansion that we began to have trouble.” In effect, Redman did not trust Rochefort’s judgment enough to be receptive when Rochefort used his intuition to fill in the many blanks in decrypted naval messages. Fortunately, Rochefort found a more sympathetic audience for his assessments in Chester Nimitz. Though Rochefort was under the administrative command of the 14th Naval District and reported officially to Redman in Washington, his mission made him invaluable to CinCPac, and in the end it was Rochefort’s relationship with Nimitz, not the one with Redman, that proved crucial.⁸

Besides Safford’s dismissal, another change was wrought by the onset of war. On December 17, Rochefort finally received authorization to drop the unprofitable pursuit of the Japanese admirals’ code and join in the common effort to crack the Japanese Navy’s far more widely used operational code, often referred to as the “five-number code.” Nimitz wanted him to pay particular attention to the “deployment of enemy carrier strike forces.” Soon the team in the dungeon began to squeeze bits and pieces of intelligence out of the messages.⁹

Back in the 1920s, when the Americans first began to pay serious attention to the Japanese naval code, they dubbed it JN-1 (Japanese naval code, version one). Over the years, the Japanese regularly changed their codes, and every time they did so, the American code breakers had to start over again. In June of 1939, the Japanese adopted a new and more complicated system. Since it was the twenty-fifth version of the code, it was dubbed JN-25. Then on December 1, 1940, the Japanese modified that code yet again, and this new variant was labeled JN-25b. It resisted the code breakers right up to the day of Pearl Harbor. When Rochefort’s team received authorization to turn their efforts to this code, they attacked it with a vengeance.¹⁰

The JN-25 b code consisted of 40,000 to 45,000 five-digit number groups, such that the messages that went out over the air waves looked something like this:

48933    19947    62145    02943    20382    16380

Some of the number groups were dummies, or fillers, added to confuse the code breakers. In addition to that, however, before sending a message, the Japanese enciphered the code again by using a cipher tablet. The encoder selected a five-digit number from this tablet and added it to the first number group in the message; the next cipher number was added to the second number group, and so on throughout the message. An indicator buried in the message itself revealed the exact location—page number, column, and line—where the cipher number additives could be found in the secondary tablet. Thus the code group for “east” might be 10236, but it would be encrypted again by adding another five-digit number from the cipher tablet. If the encoder added the number 45038, the word “east” became 55264. (Note that in adding the two numbers, there was no carrying: although adding 8 and 6 yields the number 14, only the second digit was used in the product.) To decrypt the message, the recipient needed the initial code book, the secondary code tablet, and the indicator, showing how to subtract the second from the first. The puzzle, in short, was extraordinarily complicated, which is why the Japanese remained confident that their radio messages were secure. In May 1941, when Japanese officials conducted a review of their message security, they concluded: “We need not worry about our code messages.”¹¹

Breaking through these layers of secrecy was tedious. It was helpful that the Japanese ensured that all of the original number groups were divisible by three. The reason for this was to let the recipient know he had subtracted the correct cipher—if the final code number was not divisible by three, he had probably made a subtraction error. Of course, this also allowed the code breakers to know if they were on the right track.

In addition, the volume of message traffic in JN-25 ballooned after the war began, giving the analysts more opportunities to divine the structure of the code. And finally, for a few weeks the Japanese sent messages in both the JN-25 code and the new JN-25b code because some commands had not yet received the new codebooks. This allowed the Americans to compare the messages. Station Cast identified two messages—one that was encrypted and another that was sent out in plain language—that appeared to be identical. It was the Rosetta Stone of naval messages, and it allowed the Americans to verify several of their guesses. Despite that, there were few such “aha!” moments at Station Hypo, and lots of tedious and often unrewarding analysis—plus some educated guesswork.

Because there was a shortage of personnel at Hypo, men frequently worked twelve-hour shifts, or longer. Only about 60 percent of all the messages that were intercepted could be subjected to analysis at all because there were so many messages—five hundred to a thousand every day—and breaking them took time. Of those that were analyzed, fewer than half yielded any useful information, and within those only small fragments, perhaps 10–15 percent, might be rendered comprehensible. Often the code breakers at Hypo could determine the sender and the recipient, and perhaps one or two other phrases. Here, for example, is an actual decrypt from May 5, 1942:

“KAGA and (blank) (blank) less (blank) and (blank) will depart

Bungo Channel (blank) May 4th and arrive (blank) (blank).”

It was Rochefort’s job to fill in those blanks. To say, then, that the Americans were “reading” the Japanese message traffic is an exaggeration. After much hard work, they might in the end be able to decipher a tiny fraction of it, and they had to rely on their experience, informed guesswork, and intuition to determine what it might mean and how to take advantage of it.¹²

Rochefort and his team worked long hours and with great intensity. Unable to tell whether it was night or day in their windowless quarters, they ignored the clock and often worked all night. It was routine for many of them to work twenty hours or more per day. Even the “roofers” worked watch and watch: twelve hours on, twelve hours off. One member of the Hypo team, Lieutenant Jasper Holmes, later wrote, “Had I not witnessed it, I never would have believed that any group of men was capable of such sustained mental effort under such constant pressure for such a length of time.”¹³

Because air conditioners were needed to protect the IBM machinery, it was cold in the Dungeon. Ensign Donald Showers recalled later that “it was cold as hell down there.” To ward off the chill, Rochefort often wore a maroon-colored corduroy smoking jacket over his uniform. To protect his feet from the hard concrete floor, he wore slippers. This has led some to conclude that he was highly eccentric. In the 1976 film Midway, Hal Holbrook portrayed him as a kind of cheerful goofball. In fact, Rochefort, by now a full commander, was a serious-minded and entirely professional naval officer. Asked about the smoking jacket after the war, he replied simply, “It was a practical matter, and I was just cold.” He often slept on a cot in the Dungeon instead of heading back to his lonely quarters. (His family had been evacuated back to California.) In part, his intensity derived from the nagging sense of guilt—that if only he had had access to the JN-25 intelligence before December 7, he might have been able to predict the raid.¹⁴

At his desk, Rochefort laid out the pieces of message traffic that he or someone else on his team had been able to decrypt. “You see a whole lot of letters and a whole lot of numerals, perhaps in the thousands or millions,” Rochefort recalled after the war, “and you know that there is a system in there, and there’s a little key to the system that’s something real simple, and you just keep after it until you finally solve it.” Another team member recalled, “We went over the papers one by one, we went through the whole compilation of traffic analysis, how each command, or unit, became associated with others.” Eventually, by matching number to number, phrase to phrase, and unit to unit, Rochefort could begin to assamble a bigger picture. One officer likened it to being able to visualize the overall pattern of “a Virginia reel or square dance.”¹⁵

For several weeks after Pearl Harbor, Rochefort and Hypo confined themselves to providing raw data about fleet movements and communications activity, the result, perhaps, of their intense disappointment that they had failed to predict the raid. Then in January, 1942, Rochefort noted that several of the messages he and his team were working on contained the code group that he believed stood for koryaku butai (invasion force), and that some of those same messages also contained the letters “RR,” which he believed stood for Rabaul. (In the Japanese system, all geographic locations were assigned a two- or three-letter code.) Based on that, and the overall pattern of message traffic, Rochefort predicted that the Japanese would invade Rabaul in the third week of January. When the Japanese went ashore there on January 23, it seemed proof of Rochefort’s ability to produce substantial intelligence out of a few scraps of radio traffic, and it helped lay the groundwork for a partnership of trust that soon emerged between Hypo and CinCPac—that is, between Rochefort and Nimitz.¹⁶

 Lieutenant Commander Edwin Layton was Chester Nimitz’s intelligence officer. Layton briefed his boss every morning at five minutes to eight, passing along whatever information the Hypo team had managed to cull from the airwaves. (U.S. Naval Institute)

The man who acted as the liaison in that partnership was Edwin Layton, a 39-year-old lieutenant commander with dark curly hair, thick glasses, and prominent ears. He looked more like a high school math teacher than a naval officer. After graduating from Annapolis in 1924, Ensign Layton had been assigned to escort a group of visiting Japanese naval officers around San Francisco, and he was surprised to discover that they all spoke perfectly colloquial American English. He wondered how many American naval officers spoke Japanese, and when he learned that the answer was none, he wrote to the Navy Department, deploring this fact and volunteering to become the first. At the time, Navy regulations stipulated that in the entire U.S. Navy, only two officers at a time could be assigned to language studies, and, in any case, no one could apply for it until he had completed five years of sea service. Five years later, after serving aboard the battleships West Virginia and Pennsylvania, Layton applied again. This time he was accepted. While crossing the Pacific en route to Tokyo for his new assignment as a Japanese-language officer, he met another young officer bound on the same mission. It was Joe Rochefort.¹⁷

While in Japan, Layton and Rochefort studied not only the language but also the culture. After the few hours of formal classroom study, they went out into the streets to strike up conversations. “I was most interested in why [the] Japanese do certain things they way they do,” Layton recalled, “why they think the way they do—why they approach a problem the way they do.” What both men learned was that in Japanese culture, as well as in their language, “there is more nuance than directness.” Even if the words were clearly understood, they might not reveal the true meaning of any given statement.¹⁸

Layton went back to sea in 1939 as the commanding officer of the destroyer-minesweeper USS Boggs, but in February of 1941 he was assigned to Kimmel’s staff as his intelligence officer. After Nimitz took over as CinCPac, he told Layton, “I want you to be the Admiral Nagumo on my staff, where your every thought, every instinct, will be that of Admiral Nagumo’s; you are to see the war, their operations, their arms, from the Japanese viewpoint and keep me advised about what you (as a Japanese) are thinking.” Of course it was not Nagumo that Layton should have been channeling, but Yamamoto. Nagumo, as we have seen, was merely a link in the chain, and not a particularly imaginative one. Yamamoto was the prime mover. Still, Layton got the idea. It was his job not only to monitor whatever fragments of information Rochefort’s hard-working team was able to glean from the Japanese message traffic but also to draw conclusions about what they meant as well. Soon a regular routine evolved in which Rochefort talked to Layton, often several times a day, on a secure phone line, summarizing what he had found and what he thought it meant, and then Layton would go see Nimitz.¹⁹

Layton briefed Nimitz every morning at precisely five minutes to eight. If a message came in that suggested a special urgency, Rochefort would call Layton or send a messenger to his office. If it was something of particular significance, Layton would go to fleet headquarters early or, more rarely, show up in the middle of the day. When that happened, Nimitz would interrupt whatever he was doing to see him. In effect, Rochefort was the cryptanalytic scientist doing the lab work in the Dungeon; Layton was the spokesman whose job it was to convince Nimitz to trust Rochefort’s conclusions. In Australia, the head of Station Cast, Lieutenant Rudolph Fabian, provided similar intelligence briefings for Admiral Leary and General Douglas MacArthur, yet without the kind of mutual trust and confidence that emerged in Hawaii.²⁰

One problem in the command relationship was that technically Rochefort did not work for Nimitz but for the Commandant of the 14th Naval District, and he reported to Redman in Washington, where a new office called Combat Intelligence (OP-20-GI) was supposed to collect the data and do the analysis. Layton, who was on Nimitz’s staff, was not in this chain of command. But because Rochefort and Layton were such good friends, based on their years together in Japan, there was a strong sense of partnership to their efforts. Then, too, Rochefort believed that he was uniquely placed to provide both the raw data as well conclusions about what it meant. “I felt that I had the knowledge and experience of being able to estimate and form a judgment on what [the] traffic actually meant,” he said later. “I was in a better position to say what they meant than anyone else.” As a result, the Layton-Rochefort partnership effectively bypassed Washington and took intelligence estimates directly to the theater commander, a practice that Redman increasingly deplored and resented.²¹

The unrelenting work schedule yielded results. In February the team at Hypo achieved a kind of breakthrough, and soon they were filling in many more blanks in the Japanese message traffic. By April, they were often able to intercept, decrypt, and translate Japanese messages within hours of the original transmission. On April 5, three days before Halsey and the Enterprise left Pearl Harbor to join the Hornet en route to Tokyo, Rochefort was working on an operational message that had been sent from Combined Fleet headquarters at Hashirajima. It was addressed to the aircraft carrier Kaga, still undergoing repairs at Sasebo. One number group in the message stood out. Rochefort had already determined the code for “invasion group,” and now he saw that code used in close association with the letters “MO.” Rochefort suspected at once that it referred to Port Moresby. The Japanese had used a variety of other geographical designators for Moresby, including RZ, RZQ, and RZP, and all of these had appeared with increasing frequency in messages from Inoue’s Fourth Fleet. Now, with this new intercept, Rochefort concluded that the Japanese were planning an invasion that would involve the Kaga and at least one other carrier, initially misidentified as the Ryūkaku, though it subsequently proved to be the small carrier Shōhō.²²

Rochefort called Layton on the secure phone and told him that he had “a hot one,” and that he was sending the raw decrypt over by messenger. “It looks like something is going to happen,” Rochefort told him, “that the man with the blue eyes will want to know about.” When the courier arrived, the many blanks in the message left its meaning ambiguous to a nonexpert. Though it was clear enough to Rochefort, anyone not versed in reading such messages would conclude that it was hardly a smoking gun. Over the next several days, however, more clues arrived. All that week, the men of Station Hypo focused on the growing pile of evidence that the Japanese were about to launch an offensive through the Coral Sea to Port Moresby. Holmes recalled that “the chart desk was strewn with charts of New Britain, New Guinea, and the Solomon Islands.” After the British station in Colombo, Ceylon, intercepted a message that referred to special orders for Carrier Division 5—the Shōkaku and Zuikaku—Rochefort’s team studied the traffic for any reference to those two carriers. “Each incoming message was quickly scanned for references to [Inoue’s] Fourth Fleet or Carrier Division 5,” Holmes recalled. “We lived and breathed and schemed in the atmosphere of the Coral Sea.”²³

It paid off. By midmonth, the stream of messages indicated quite a bit about both the target and the time frame, though it was less clear which units would participate. The Japanese were evidently planning a major operation in the South Pacific. There were references to at least four carriers, two cruiser divisions, and a destroyer squadron, plus various landbased units. Altogether nearly three hundred surface units appeared in the message traffic—the largest assembly of warships in the war to date. In this regard, it was unclear what Nimitz could do about it. By now both the Enterprise and Hornet were beyond recall, more than halfway to Tokyo with Doolittle’s bombers. That left only the Yorktown, still in the South Pacific after the Lae-Salamaua raid, though in serious need of a refit and resupply, and the Lexington, which was in Pearl Harbor having her big eight-inch guns removed and replaced by antiaircraft guns. With the departure of Vice Admiral Wilson Brown for San Diego, the Lexington task force was now under the command of Rear Admiral Aubrey “Jake” Fitch, a short, broad-shouldered brown-shoe officer who had earned his gold wings in 1930. Even assuming that Fitch’s Lexington group could get to the Coral Sea in time to join the Yorktown, the two American carriers might prove insufficient to interfere with the thrust, given the size of the Japanese commitment. At an April 18 staff meeting, the general agreement at headquarters was that “CinCPac will probably be unable to send enough force to be sure of stopping the Jap offensive.”²⁴

The next day, Doolittle’s bombers completed their mission over Japan’s cities, and Halsey’s two carriers began steaming back toward Pearl, though it would take them a week to get there. Layton briefed Nimitz on April 22 that Rochefort’s intercepts offered clear “evidence of a powerful concentration in the Truk area,” and he suggested that “this will be the force which will make the long expected attack to the Southwest.” Jasper Holmes urged Rochefort to tell Nimitz that he should order Halsey not to return to Pearl at all but to refuel at sea and steam directly south to the Coral Sea. Rochefort reminded the enthusiastic Holmes that it was not the place of Navy lieutenants, or commanders for that matter, to tell four-star admirals what they should do. Their job was to provide the information that would allow the admiral to make his own decisions.²⁵

Nimitz did not order Halsey to steam southward; the logistic realities made such a decision impossible. Nonetheless, he did trust Rochefort’s analysis and, despite the odds, was determined to commit his other two carriers to confront the Japanese offensive. He notified King that it was his “strong conviction” that the Japanese thrust toward Port Moresby “should be opposed by [a] force containing not less than two carriers.”²⁶

The message triggered alarms in Washington. Redman remained distrustful of Rochefort and the assessments of Hypo. Station Cast, by now removed to Melbourne, Australia, and referred to as Belconnen, reported the Japanese objective as “RO,” not “MO,” and suggested that the target might be the Aleutian Islands rather than Port Moresby. If Rochefort were mistaken, Nimitz would be sending his last two carriers in the wrong direction. Though Rochefort was able to demonstrate that Belconnen had incorrectly decrypted the code, Redman remained skeptical. In fact, he was more than a little annoyed that Rochefort had bypassed him by taking his analysis directly to Nimitz. Redman wanted all intelligence intercepts to be sent to OP-20-GI in Washington, interpreted there, and then disseminated out to the fleet commanders. Rochefort should confine himself to purely tactical matters while Washington dealt with the broader strategic questions. Redman couldn’t complain about this to Nimitz. He did, however, express his doubts to King.²⁷

With Nimitz urging instant action and Redman expressing skepticism, King took the unusual step of writing directly to Rochefort to ask for “Station Hypo’s estimate of … future Japanese intentions.” In effect, King wanted Rochefort to defend and justify his assessment.

Rochefort wired back his response only six hours later (with a copy to Nimitz) in a concise report that made four main points:

1. The Kidō Butai was in the process of withdrawing from the Indian Ocean, and its next effort would be in the Pacific.

2. The Japanese did not plan to invade Australia.

3. A new plan of operations involving some, but not all, of the Kidō Butai was preparing to strike southward from Rabaul through the Coral Sea toward Port Moresby.

4. There were hints of another, even larger operation that would take place after Port Moresby, though its scope and objective were not yet clear.

The summary was remarkable for both its candor and its accuracy, and it convinced King that Rochefort knew what he was talking about. King even suggested that the American force in the Coral Sea might be bolstered by sending several American battleships there—the rehabilitated survivors of the Pearl Harbor attack.²⁸

Nimitz, too, accepted Rochefort’s conclusions, but he was skeptical that sending battleships to the Coral Sea offered any kind of solution. He thought the battleships too slow, too vulnerable, too difficult to keep full of fuel, and in any case unlikely to affect the balance of power in the South Pacific. Nimitz ordered them back to the West Coast, mainly to get them out of the way. Instead, he would pit his two carriers against the three (or possibly four) carriers that the Japanese committed to the operation. He was willing to accept the odds because of, in his words, “the superiority of our personnel in resourcefulness and initiative, and of the undoubted superiority of much of our equipment.” It was still remotely possible that Halsey could get to the Coral Sea in time, but even if he couldn’t, Nimitz was determined to oppose the Japanese thrust anyway. He ordered Fletcher to head for Noumea to restock the Yorktown’s near-empty larder and equip the Wildcat fighters with new self-sealing fuel-tank liners. Meanwhile, Fitch’s Lexington task force would steam from Pearl Harbor for the Coral Sea. The two carrier groups would rendezvous on May 1, at which time they would constitute “a single force under [the] command [of] Rear Admiral Fletcher.”²⁹

That, too, worried King, who remembered that it was Fletcher who had commanded the failed relief expedition to Wake, and that Fletcher had accomplished little during the raid in the Marshalls in early February. Just a month earlier, King had seen a copy of a message Fletcher had sent to Nimitz informing him that he was “en route [to] Noumea … for provisions.” Without consulting Nimitz, King shot back: “Your [message] not understood if it means you are retiring from enemy vicinity in order to provision.” The pugnacious King declared that the men should live off hardtack and beans as long as they could still fight. In fact, the crew of the Yorktown had already been eating emergency rations—mostly beans and canned spinach—for several weeks. When there were only five steaks left on board, Captain Buckmaster had raffled them off, drawing the winners’ names from a hat. King was unimpressed; during those same weeks, after all, the Yorktown had not struck a blow. Now he worried that Fletcher would not be sufficiently aggressive commanding a two-carrier task force in the Coral Sea. To discuss it, King asked Nimitz to meet him in San Francisco, and Nimitz flew there on April 24.³⁰

At their meeting, Nimitz summarized his plan to concentrate all four American carriers in the Coral Sea, though he acknowledged that since Halsey could not get there until May 13 at the earliest, it meant that Fletcher’s two carriers would very likely have to take on at least three, and perhaps as many as five, Japanese flattops. King again raised the idea of sending some old battleships there; Nimitz gently deflected the suggestion. They also discussed Fletcher’s command temperament. Both men expressed “uneasiness as to Fletcher’s operations,” but short of flying Halsey down there to take over Fletcher’s task force, which would be awkward in the extreme, there seemed to be no alternative. King reminded Nimitz that April 29 was the emperor’s birthday and suggested that the Japanese might have some special operation planned to coincide with that date. Nimitz acknowledged that, but based on Rochefort’s analysis he remained convinced that the operation would not begin until May 3. Nimitz flew back to Pearl on April 28 not knowing King’s final decision. When he arrived, he was gratified to find that King had approved his plans. Halsey would return to Pearl, resupply his task force, and then take it south to join Fletcher as soon as possible. If Halsey did not get there in time, however, which seemed likely, Fletcher would simply have to do the best he could.³¹

The very day that Nimitz flew to San Francisco, Rochefort’s team broke another message from Admiral Inoue himself. As usual, there were many elements of the message that could not be read, but it contained call signs for “the MO fleet” and “the MO attack force,” as well as “the MO occupation force.” Another message contained code groupings for the Shōkaku and Zuikaku as well as other ships. Rochefort was able to tell Layton, who then told Nimitz, that a Japanese invasion force with at least one carrier and possibly more was planning to enter the Coral Sea around the eastern end of Papua/New Guinea on or about the third of May, and that another task force that included the Shōkaku and Zuikaku would provide cover for the operation. Rochefort reported that X-Day, the day scheduled for the landing at Port Moresby, was May 10.³²

Halsey’s Task Force 16 left Pearl on April 30.* It had taken five days to resupply his two big flattops and their escorts not only with oil but also with ammunition, beef, and beans (and presumably canned spinach). That same day (May 1st west of the International Date Line), the Shōkakuand Zuikaku left their base at Truk in the Caroline Islands almost due north of Rabaul. Since they had to travel less than a third of the distance that Halsey did to reach the Coral Sea, it was now certain that Halsey would not get there in time. For better or worse, the defense of Port Moresby would depend entirely on the Lexington and the Yorktown, their very names evoking the alpha and the omega of the American Revolution, with both of them under the tactical command of Rear Admiral Frank Jack Fletcher.³³

* The “on-the-roof gang” got its name from the fact that the men were trained on the roof of the Navy Department Building in Washington on which the radio intercept tower was located. Because the building had not been designed for such use, the trainees had to climb a ladder to get there.

* The term Hypo (the British phonetic code for the letter H) derived not from its location in Hawaii or Honolulu but from the radio tower at He’eia. Hypo was later redesignated as the Fleet Radio Unit, Pacific (FRUPAC). Station Cast was named for Cavite Navy Yard in Manila and later moved to Corregidor when the Japanese overran Manila. Eventually it relocated to Melbourne, Australia, where it was renamed Belconnen, and later Fleet Radio Unit, Melbourne (FRUMEL).

**In the interest of accuracy, not to mention fairness, it is important to note that one of the prime movers of this early organization was Agnes “Miss Aggie” Driscoll, who began working in the Code and Signal Station when Safford did in 1924. She held the rank of chief yeoman (the highest then available to women), though she all but invented the science of cryptanalysis and trained most of the men who later played such a crucial role in American code breaking.

* That same day, the Japanese changed their code system for geographical designators. They often did this just prior to a new operation to ensure secrecy. This time, however, their decision backfired, since the change helped confirm the imminent attack, and the message contained both the old and new designators, which allowed the code breakers to update their dictionary of designators.