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The Battle of Midway
How Cryptology enabled the United States to turn the tide in the Pacific War.
A Precarious State of Affairs
While Yamamoto plotted to bring a quick end to war in the Pacific Theater, the United States Navy in the Pacific, led by Admiral Chester Nimitz, was desperately trying to anticipate Japan's next move. Nimitz, unlike his counterpart, had little room for error. At the time of the battle, his 3 aircraft carriers, 45 fighting ships, and 25 submarines were all that lay between Hawaii and the West Coast and a large Japanese Fleet that had yet to suffer a significant defeat. It appeared that Nimitz would have one shot at the enemy. A miscalculation by Nimitz on where Yamamoto would strike next would not only be disastrous, but also possibly fatal to the Allied war effort in the Pacific.
In order to prevail, Nimitz had to have some sense of Japan's intentions. The task of obtaining the critical information required to turn the tide in the Pacific fell to OP-20-G, the Navy radio intelligence organization tasked with providing communications intelligence on the Japanese Navy. Established in the early 1920s by Laurence F. Safford, the " Father of Navy Cryptology," OP-20 -G was key to Nimitz's planning. In addition to his earlier cryptologic efforts, Safford had played a major role in placing Commander Joseph Rochefort in command of Station Hypo, the Navy's codebreaking organization at Pearl Harbor. Over a period of 18 years, OP-20-G had developed a highly skilled group of officers and enlisted men.
In 1942 Rochefort and his staff began to slowly make progress against JN-25, one of the many Japanese command codes that had proven so challenging to the Station Hypo team. JN-25 was the Japanese Navy's operational code. If it could be broken, Rochefort would be able to provide Nimitz the information he needed to make wise and prudent decisions concerning the dispersal of his precious naval assets.
The Breaking of JN-25
AF Is Short of Water
In an effort to alleviate any doubt, in mid-May the commanding officer of the Midway installation was instructed to send a message in the clear indicating that the installation's water distillation plant had suffered serious damage and that fresh water was needed immediately. Shortly after the transmission, an intercepted Japanese intelligence report indicated that "AF is short of water." Armed with this information, Nimitz began to draw up plans to move his carriers to a point northeast of Midway where they would lie in wait. Once positioned, they could stage a potentially decisive nautical ambush of Yamamoto's massive armada.
Due to the cryptologic achievements of Rochefort and his staff, Nimitz knew that the attack on Midway would commence on 3 June. Armed with this crucial information, he was able to get his outgunned but determined force in position in time. On 4 June the battle was finally joined. The early stages of the conflict consisted of several courageous but ineffective attacks by assorted Navy, Marine, and Army Air Corps units.
The tide turned however, at 10:20 a.m. when Lt. Commander Wayne McClusky's Dauntless dive bombers from the USS Enterprise appeared over the main body of the Japanese invasion force. After a brief but effective attack, three of the four Japanese carriers, the Akagi, Soryu, and Kaga were on fire and about to sink. Later that day, Navy dive bombers located and attacked the Hiryu, the fourth and last major carrier in the invasion force, sending her, like the previous three, to the bottom.
In the end, Yamamoto's worst fears had become a reality. Due to an impressive mix of leadership, determination and skill on the part of Admiral Nimitz, the officers and men of Station Hypo, and the pilots soldiers, sailors and marines who carried the fight to the enemy, Japan would be on the defensive for the rest of the war. The Rising Sun of Dai Nippon, which had shone so brightly for so many months, was beginning to set.
-- Patrick D. Weadon
Historical Document | Date Posted: Jan 15, 2009