In the early months of 1942 Admiral Isaroku Yamamoto, Commander of the Japanese Imperial Navy hoped to recreate the crushing victory of Pearl Harbor by attacking the Allied base at Midway Island in the Pacific. This is emphasized after the US Pacific Fleet dealt a blow during the Battle of Coral Sea in May 1942. At Coral Sea the US Navy intercepted JPN codes and it enabled them to send aircraft carriers to the area, joining the Australians to oppose the Japanese. It is of special interest to note that the Battle of Coral Sea is memorable due to it being the first battle in history to involve only aircraft carriers. Though a Japanese victory in terms of damages, the strategic victory of the Allies would lead to success at Midway. Due to the USN cryptanalysts who began working through Japanese codes in early 1942, the Pacific Fleet knew of the attack, at a location referred to as “AF” by the Japanese weeks ahead of the June battle, thus were able to confirm the location “AF” as Midway by means of deception. When suspecting Midway as “AF” the base sent out a dummy request for water resupply with intentions of the Japanese listening in on the transmission. The Japanese received the message and began planning for an attack at “AF”, thus it was confirmed Midway was indeed “AF”. This breakthrough in cracking Japanese codes would prove fatal for Yamamoto and the Empire of Japan due to the efforts of USN intelligence officials. By June 6th, 1942 the Japanese Navy retreated and remained largely on the defensive the remainder of World War II.
Throughout recorded history, coded messaging has always served the purpose of understanding the intentions of an opposing force. For strategic military advancement this was without question, of the upmost importance. Never in the history of global conflict had naval and aerial warfare dominated as they did in the Pacific Theater of World War II. Through cryptanalytical development the U.S. snatched the planned surprise attack of Midway and its surrounding islands from Imperial Japan and turned the tides of war. How had USN cryptanalysts on the eve of December 7th, 1941 been unable to decode warning signs of Pearl Harbor, yet a mere six months later prevented a devastating blow to the remnants of the U.S. Pacific Fleet and turn the tide of war? The answer is still the subject of much debate and offers varying degrees of interpretation and understanding.
The study of signals intelligence came to prominence during World War II, but its inception in the modern sense began in the Great War, though the basics of reconnaissance and surveying began long before. As early as the 1920s, the United States began attempting to crack Japanese Imperial naval codes. It would not be until the aftermath of Pearl Harbor that the United States truly began to make progress in deciphering Japanese codes. However, what limited readability the cryptanalysts had were invaluable in the months after official entry into the war.
There is much on the subject of the United States entry into World War II, and its dramatic conclusion with the use of atomic weapons in Hiroshima and Nagasaki in 1945. However, there have been expansions upon interpretations, and broader context as well as more and more declassified material being released as time goes on. It is worth noting that often the Pacific front is seconded to the European Theater, both in terms of public understanding and scholarly studies due to the popularity and romanticism of war in Europe. Contributing to this thought is the larger access of information both on the front of the Allies and the Axis powers in Europe, whereas Imperial Japan was innately more secretive and reclusive with their documents. Historical documents are often more readily available by the victorious, as many historians are fully aware, and the Pacific Theater is no exception to this. There are authors, such as Ronald H. Spector, author of Eagle Against the Sun (1985), and David Kahn, author of The Codebreakers (1996) and Seizing the Enigma (2012), who offer a popular comprehensive look into the war as it pertained to US/Japan relations, which has caused renewed interest in the topic of intelligence development during World War II. Spector’s choice of interpretation that, regarding Pearl Harbor, “frequently army and navy leaders discussed the possibility of Japanese attack, in the final analysis it appeared unlikely, almost fantastic” seems contrite and bland. Though it is fair to say that he draws this conclusion based on preliminary intelligence reports that the Japanese would first draw upon the Dutch and British forces of Southeast Asia. As he states in his work, few expected the Japanese would make a simultaneous bid for Pearl Harbor, even if they could have, due to the Pacific Fleet, while formidable, was “in no shape to contest Japanese moves in the western Pacific.” While this was an interpretation for the situation in the early period before the United States officially entered World War II, current interpretations of documents show how critical of an error this would prove to be. As early as 1930, Japan and U.S. diplomatic relations were strained, but by the summer of 1941 these relations reached a diplomatic impasse. The U.S. had multiple intelligence stations by this time, each tasked with decrypting military codes.
The United States has never officially published a history of its cryptoanalysis operations during World War II, neither have many other nations. There is still much that is classified, though many items are slowly becoming declassified and available for study. One such work, an anthology of military intelligence written by Jack Finnegan and Jim Gilbert, U.S. Army Signals Intelligence (1993), takes a factual approach to Allied intelligence successes in the Pacific. Finnegan and Gilbert’s interpretation of information is based upon released volumes from the U.S. Department of Defense, in conjunction with excerpts from letters and communications within top officials of the period.
Timothy Wilford, author of the article ”Decoding Pearl Harbor” (2002), aligns with the defense that the reason cryptanalysts didn’t prevent Pearl Harbor was emphasized “that any forewarning of the Pearl Harbor attack was not possible”. Wilford also aligns with David Kahn’s thoughts that what he considered the US Navy’s poor codebreaking was “due less to Japanese cryptographic superiority then to the Navy’s insufficiency of cryptanalysts”, and yet another preliminary agreement to the subject surfaced in 1967 with author Ladislas Farago. Farago had a rather generalized account due to classified sources at the time of his analysis in that he too argued “the USN cryptanalysts were not particularly effective by late 1941”.
- Development of coded messages:
World War I saw significant changes and evolution in espionage and intelligence collection operations, as well as the creation of sizeable staffs both in the field and national capitals to make and break code and cipher systems. When the Great War began, the few intelligence personnel who existed still thought of military intelligence in the sense of the Franco-Prussian War. Early World War I intelligence officers like Dillwyn Knox who was one of Bletchley Park’s important members during World War II until his death, and William Friedman who’s comparable achievements contributed to American signals intelligence on a large scale were instrumental in the evolution of modern intelligence. It would be the 1920’s that would see US Intelligence focusing on Japanese decryption with the encoded info of the Red Book, and later Blue Book, which took until the late 1930s for Intelligence officials to decode. However, by this point Japan had already moved on to a new codebook, later called the JN-25B. No matter how the US attempted to intercept messages, they simply did not have the resources to effectively decrypt the JN-25 codebook at a pace useful to growing political tensions.
- Political Tensions
From December 1937 to January 1938 hundreds of thousands of Chinese civilians would be murdered, tortured and raped by Japanese invading forces in what became known as the Nanking Massacre, also condemned as the “Rape of Nanking”. The Japanese would carry out this order under General Iwane, commander of the Japanese Army in Central China, who also commanded his troops to loot, destroy and burn not only the city but surrounding towns and villages. Allied nations condemned the act as inhuman and horrendous, and as such General Iwane would be tried and executed as a war criminal after the end of the War. The Nanking Massacre was a major contributing factor to declining relations between the United States and its trade partner. The Japanese were far from satisfied with claiming just China for their own, they wanted to build a global empire, and China was just the start. Japan’s actions against China and its surrounding nations would be a prime cause for the US pulling trade support for the oil, rubber, and tin, all of which Japan desperately replied on desperately, and could only acquire through trade. This was also a period in which the Middle East had not yet become a global producer of oil, therefore Japan had little ability to secure other trade without the US being their main supplier. Japan became more aggressive in their claims of expansion as trade embargos and restrictions continued throughout the 1930s. In 1939 the US officially ended their trade agreement with Japan, stating that they would no longer offer items to the country that could be used for war.
Japan itself was also in turmoil within as a new “moderate” government had been set up due to passive alliances within the Allies and Axis countries internally. Japan saw this as ample opportunity to create a more progressive government itself. However, it did not last long and a group of extremists easily overthrew this new moderate government, and in doing so secured Japan’s alliance with Nazi Germany. General Tojo, the new Minister of War, almost immediately demanded Britain turn over some of its territory in Indochina (present day Vietnam), and also close down the Burma Road which was used for trade by many, especially the US which supplied munitions to China through the Lend-Lease Act. It would be this that would begin the final nail in the coffin for US-Japan relations.
In September 1940, Japanese forces invaded Vietnam as they saw its location as a prime area to attack Allied forces in the Southeast Asia region, as well as block US supplies to China. Japan sent what they called a request, but truly it was more of an ultimatum, that the French, who had fallen to Germany in June of that year, allow the Japanese to occupy their territory and block supplies into China. They were successful. However, the Japanese did not adhere to their end of the bargain and only occupy this area for the sake of trade prevention. The Japanese would do this continuously throughout the war. This agreement stated that the Japanese could occupy a certain portion of northern Indochina with no more than 25,000 Japanese soldiers back and forth, and only 6,000 stationed in the area at any given time. However, in late September the Japanese continued their expansion on three fronts, taking the railway at Lang Son, thus breaking their agreement with the French as this was outside the agreed territory. While the French protested the breach of agreement, the Japanese attacked the French by air and land. The French units in the area held their ground for two days before falling to the Japanese who then claimed the area. The Japanese would issue an apology to the French for this incident, and free their captive prisoners, but a large portion of Indochina would remain largely under Japanese control for the remainder of the war.
Due to continuous incidents like these, much of what would eventually form the entirety of the Allied nations began to severely criticize Japanese methods and expansion. However, the Japanese government was apparently oblivious to the criticisms of what where once allies to the small island. The breakdown of relations followed rapidly once the US sanctioned embargoes, and publicly shared its disdain for the way the Japanese committed massacres across Asia in their quest for expansion. In fact, British Prime Minister, Neville Chamberlain, believed the only possible military solution to the Japanese problem lay in joint Angelo-US action but President Roosevelt rejected the idea and the British were not prepared to go it alone. While the US continued its isolationism it was apparent in Washington that war was all but inevitable.
Michael Smith states the US Army SIGINT was known for their pacesetting in the race to break Japanese codes, but it was not until 1936 that they were able to crack the JPN Red Book, and it would not be until 1938 that Signal Intelligence personnel cracked the Blue Book, both of which were first intercepted many years before. It would be even longer still before the US would duplicate the British deciphering machine, known as the J machine. During this period, the US 442nd Infantry Regiment became secretly one of the most important contributions of intelligence operations. This detachment was comprised almost entirely of second-generation Japanese Americans who faced discrimination after Pearl Harbor, not only from civilians but also from their fellow soldiers. Despite their treatment these soldiers would continuously work not only in the battlefield, but their best and brightest contributed to decoding JPN codes in secret. The 442nd has been noted as one of the most decorated units in American history for their importance, and their valiant determination contributed to the victories in the Pacific.
- Pearl Harbor
Even before war officially broke out between the United States and Japan, the US had been preparing for a possible war scenario due to these tensions. In fact, the United States boasted a large and plentiful Navy design, however the problem resided in the fact these newly designed ships and aircraft would not be available until after the entry into the War. Most would not be readily available for deployment until 1942 and 1943. However, in early 1941, the USN was still a formidable force. In February 1941, the USN had relocated its headquarters to Pearl Harbor, Hawaii, also called STATION-HYPO. STATION-HYPO was one of the Navy’s primary decryption designations tasked with intercepting and deciphering Japanese radio transmissions.
The photo above shows Pearl Harbor before the Japanese attack. At the time, the entirety of the Pacific Fleet was Headquartered here after being moved earlier in the year from its previous home in California. Hawaii was a perfect posting for many service members who called Pearl paradise, the golden assignment, and it was until that fateful morning on December 7th, 1941. Many would argue the USN made numerous critical errors on December 7th and was quite literally caught sleeping by the enemy. Nearly all servicemen were on leave that weekend, every battleship and over three thousand aircraft at their station, and as such Pearl Harbor was at its most populous…and a very easy target. The only part of the Pacific Fleet not at Pearl were the aircraft carriers who were out on exercise in the Pacific, which would prove to be a grave error on the part of the Japanese Navy intelligence. Early radar detection on the morning of the attack received warning of Japanese aircraft, however the Signal Corps did not fully investigate as they assumed they were the expected bombers from the US mainland. While STATION-HYPO was one of the leading decryption centers for JPN intelligence, the US had cracked less than 15% of the encrypted traffic codes at the time of the attack. On top of that, even if it was possible to read Japanese code, many officials have stated the high-level communications of the Japanese Foreign Office did not carry the details of the Japanese military planning. Edward Drea contends that the United States could not decrypt a single Japanese army message at the time of Pearl Harbor, and that it actually wasn’t until September 1943 did American codebreakers read their first full Japanese message. This is in direct contrast with Fleet Intelligence Officer to Admiral Nimitz Rear Admiral Edwin Layton who stated while only 10% of JPN codes were readable in 1941, they had potential intelligence value as clues to Japan’s operational intentions and were therefore excessively valuable.
The Japanese used their skillfully planned attack to strike swiftly and methodically, and almost as quickly withdrew. This fact was much to the chagrin of Admiral Yamamoto, who could not understand why Admiral Nagumo chose to withdraw instead of completely obliterating Pearl Harbor when he could have easily done so at the time. The Japanese attack stopped at wave two, though originally Nagumo intended upon three. Once the Japanese withdrew, and the smoke began to slowly clear nearly every battleship on Battleship Row was damaged or destroyed, nearly 3,000 members of the US military and approximately 1,000 civilians and thousands wounded, and hundreds of aircraft destroyed. On December 8th, 1941 the United States officially declared war on the Japanese.
- Early 1942
In the aftermath of Pearl Harbor Admiral Husband E. Kimmel, Commander of the Pacific Fleet at the time of the attack was removed from his position and replaced with Admiral Chester Nimitz. Kimmel would take the fall for the unpreparedness of the Pacific Fleet on December 7th, but whether he is truly to blame is still the topic of much debate, though many believe it simply that Washington needed a scapegoat for their failure. However, one important fact is universally recognized: The US aircraft carriers were not at Pearl Harbor and remained untouched during the attack as they were out delivering supplies to Wake Island and Midway. Aircraft carriers had already replaced destroyers as the most crucial members of the navy as World War II birthed the first true importance of aerial fighting. Had the carriers been at Pearl it is theorized the war would have likely had a very different outcome, and not in the favor of the Americans.
In January 1942, the newly appointed Admiral Nimitz directed Admiral Halsey to attack the Marshall and Gilbert islands in retaliation for Pearl Harbor. The goal was to remove the Japanese ability to gain strength in the area, as the Japanese already boasted quite a large amount of functioning submarines on the islands. In the weeks preceding the attack on the Marshall Islands, Japan had continuously had the upper hand in the Pacific, especially with fall of Wake Island just days after the attack on Pearl Harbor. The attacks on Wake Island actually began hours after the Pearl Harbor attack on December 8th, 1941 when the Japanese launched a separate assault on Wake from the air. Though there are American aircraft on patrol in the area, due to visibility issues they were unaware of the Japanese assault. American forces held out against the Japanese for nearly two weeks before surrendering on December 23rd. The majority of those captured were executed, including many civilian construction workers. The loss of Wake Island so soon after the devastating blow at Pearl Harbor cause American morale to hit an all time low, while fueling Nimitz to set his sights on Japanese controlled islands, such as the Marshall and Gilbert Islands. While reports on the Marshall Islands were by no means complete, US intelligence compiled enough that Nimitz felt his approach just. The Japanese garrisons under the command of JPN Admiral Inoue were a formidable opponent against the raiding aircraft of the Yorktown which did light damage. While the raids themselves did very little in terms of strategic impact, it did wonders for the morale of soldiers as well as the civilian population. It also paved the way for the Doolittle Expedition months later, in which two key carriers, the Hornet and Enterprise contributed, thus their absence from Coral Sea. These seemingly small raids, while strategically irrelevant, proved to be the morale boost needed to fuel a new determination in American forces. In just a few short weeks after small pinpricks in the Japanese line the Allies would get their first taste of success at Coral Sea.
- The Turning Point
The Battle of Coral Sea would be the precursor to Midway, and inevitably set the stage for the explosive and history changing battle shortly after. The initial idea was for Japan to ultimately watch campaigns into Fiji, Samoa, and surrounding islands in order to cut off Australian supply lines with the US. The Japanese assumed if they cut off Australia’s supply line the US would not only lose support in the Pacific, but Japan could expand into Australia. However, due to intelligence decryption the US Navy was aware of the attempt and was able to plan a means of preventing the JPN from achieving their goals. Japanese Admiral Yamamoto lent two of his carriers slated for the planned Midway offensive to Admiral Goto, a mistake that would cost him dearly only a few weeks later. The Japanese lost several warships, including one carrier and thus had to end their campaign. The US lost Lexington and Yorktown was badly damaged. The Battle of Coral Sea, though not effective in terms of debilitating the enemy it would cement itself as an important milestone in the USN quest to eliminate the Japanese as a threat. Coral Sea was the first battle fought by carriers alone as well as important in that it truly sealed the fate of the battleship as the desired heavy hitter in battle. The American and Japanese carriers never once set sight on one another, thus the entire battle itself was fought almost completely aerially. Following Coral Sea there was an uneasy and suspicious pause in Japanese operation in the Pacific. It was clear the Japanese were preparing to strike with the purpose of once again catching American forces off guard. The action at Coral Sea also made it clear to the Japanese that Americans had most of its carriers in the Pacific. This concentration of American naval strength in the South Pacific very probably appealed to the Japanese High Command as offering a most strategic moment for a heavy blow against American positions in the mid-Pacific.
On June 4th, 1942, three of Japan’s four aircraft carriers were damaged beyond repair in eight minutes by American aircraft. Craig Symonds, author of The Battle of Midway (2011) states that “These pivotal minutes-the most dramatic in World War II, indeed perhaps in all of American history-reversed the seemingly irresistible momentum toward Japanese victory. Symonds plainly states that the Japanese would never recover from this blow. Author Thomas Wildenberg considered Midway as being one of the most decisive battles of World War II in that it should have been a Japanese victory, but was not despite its formidable strengths, the Japanese navy committed at Midway a series of irretrievable strategic, tactical, and operational mistakes that seem almost inexplicable. Wildenberg uses many examples and factual content from varying sources that all have one fact in common: The United States should never have won Midway considering the weaker numbers and firepower against the seemingly impenetrable Japanese forces. Wildenberg theorizes this possibility to be that neither Yamamoto nor Naval GHO truly comprehended the strengths and weaknesses of the world-class weapons system [the aircraft carriers of the First Air Fleet] they possessed. It is, in fact, aircraft carriers that not only rose to prominence due to the US heavy damaged battleships at Pearl Harbor, but also dominated the Pacific theater. At Midway the aircraft carrier would claim its place at the forefront of naval warfare.
JPN intelligence stated that after Coral Sea Midway Island had one squadron of bombers, one squadron of fighters, and two squadrons of flying boats in terms of air strength. In fact, the Japanese wholeheartedly believed that they could attack Midway by air and destroy shore based air strength to facilitate landing operations and still be able to destroy any counterattack the USN could muster. In a sense the Japanese had an air of overconfidence due to their series of crushing victories. Despite Coral Sea being quite a draw between the USN and JPN, the Japanese still considered it a victory on their part. Little did the Japanese know, the USN already knew a large summary of their plans for Midway due to decryption efforts on the part of intelligence. In every sense the US should never have been victorious at Midway. The sheer thought based on all projection and evidence suggests the likelihood of victory was so slim to engage would be suicide. However, due to the efforts of SIGNIT the US was as prepared as they could have been for the days that lie ahead. The Japanese geographical designator “AF” began to appear in partially decrypted messages as early as 4 March 1942. However, Washington was not convinced this designation was Midway, instead they fully believed it may be Samoa, or even Hawaii itself. STATION-HYPO was aware of the trouble in Washington due to lack of agreement. In order to convince Washington and get the support they needed HYPO was granted approval for a ruse to essentially shut Washington up. This ruse would be a message sent from Midway complaining of a water shortage. Nimitz approved the message to intentionally be intercepted by the Japanese, who fell perfectly into his trap when they sent a decrypt from Tokyo Naval Intelligence advising of a water shortage at AF. Not a single skeptic in Washington could refute the evidence this brought.
In late May 1942 HYPO discovered a critical piece of information in their decryptions in the form of the Japanese date cipher. This meant the USN cow possessed the means to determine the final missing piece of the puzzle – when Japan would attack Midway. Thanks to the efforts of HYPO the Pacific Fleet was acutely aware how dispersed the JPN was. It would be Admiral Yamamoto’s necessity of using radio transmissions that would provide codebreakers with the intended invasion date: June 4th or 5th. While the Japanese launched a diversionary strike on the Aleutian Islands on June 3rd, Midway prepared to launch a counter strike with B17 bombers mistaking the invasion force for the main JPN fleet. With the first wave of bombers unsuccessful, the second wave was launched on the invasion force of Admiral Kondo on the morning of June 4th, which was also unsuccessful. In the early morning hours of June 4th the Japanese carrier force had not yet been located and was probably approaching from the northwest under cover of some bad weather in the area. At 0545 a fighter patrol reports back there were a considerable amount of enemy aircraft heading toward Midway, and a mere five minutes later radar detection at Midway would report the planes a mere 93 miles away. This intelligence relay allowed air raid sirens to be employed at 0555, and by 0605 every able aircraft was in the air. At 0615 the same aircraft encountered the detected Japanese aircraft and the Battle of Midway commenced. The Japanese would quickly follow with bombs against Midway, though antiaircraft were able to take down a few bombers, and the bombing was over within a few minutes of beginning, however the fighters above continued to engage. While a large majority of US pilots felt their fighters lacking in comparison to the Japanese Zeros, the determination and skill allowed for exploitation of the speedy aircraft’s vulnerabilities. The damage to Midway was insurmountable. Almost all structures above ground had been destroyed or badly damaged. The Japanese spared the runways, however, assumedly for their own use once gaining control of the island. In the face of this devastation those who resisted at Midway would find comfort in the fact that no sooner had the last Japanese aircraft departed Midway than US fighters began their assault on enemy carriers.
By 0700 bombers and fighters alike had their sights on the approaching fleet, consisting of three carriers, six destroyers, one battleship, and several cruisers in addition to the regrouped flyers. Admiral Nagumo originally planned a second wave of attack on Midway, however as he was preparing rearmament a scout plane informed him portions of the US Pacific Fleet just to the east of Midway.
The USN had an ace in the hole. Not only did Nagumo now realize the USN had prepared for this exchange of firepower, but an unexpected participant arrived, the USS Yorktown. The Japanese knew that an imperative US carrier, Yorktown, had been heavily damaged at Coral Sea, thus believed her to be unavailable for the response at Midway. This would have been an accurate assumption except Yorktown limped back to Pearl Harbor where expedited repairs were completed in a mere two days, allowing Yorktown to regroup with the Pacific Fleet near Midway in formation for the attack. Within the hour of their reported position Hornet, Enterprise, and Yorktown all began launching squadrons of bombers, torpedo craft, and support fighters. Unlike Coral Sea, both USN and JPN carriers and supporting PT boats played an integral part in damages. Though the majority of fighting would cease by the end of June 4th, skirmishes would continue for another two days. On June 6th, 1942 both the US and Japanese suffered critical losses. The US suffered the loss of Yorktown and Hamman, along with over 100 planes and more than 300 servicemen. However, the Japanese truly suffered for their part in Midway. On June 6th, 1942 Admiral Yamamoto ordered all ships to retreat. The Japanese suffered heavy losses with nearly 3,00 men, including hundreds of the best pilots the Japanese had to offer, over 300 aircraft, four of their carriers, and many other support vessels. The crushing blow at Midway broke Japanese confidence and paved the way for the US to secure the Pacific. The Japanese would never recover from their defeat. Midway effectively sealed their fate in ways that would continue to become apparent throughout decryption efforts the following year.
It is often said the act of war never changes, however that could not have been further from the truth during World War II, especially in the Pacific. The advancements in technology and cryptanalysis forever changed how wars were fought and won. Intelligence rose to a prominent status that has developed over decades, proving that even the most secretive operations are imperative to war effort. Intelligence in and of itself will not win a war, but its importance in the Pacific Theater cannot be overlooked. Inevitably it was intelligence that allowed the US to not only surprise the Japanese at Midway but enabled them victory. A Victory that would break the Japanese spirit in such a way that they would spend the rest of the war in a downward spiral toward surrender. The effort of signals intelligence officials in the early period of World War II allowed the USN to adapt to JPN tactics and strategic placement, and the members of SIGINT stations themselves propelled the US to its eventual victory in 1945 through their dedication to cracking an ever-changing Japanese naval code. While often overlooked in importance in favor of dramatic firepower and display of power, cryptanalysts may have developed as the most important members of the world’s most dangerous game.
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