Scene on board USS Yorktown (CV-5), during the Battle of Midway, 4 June 1942, shortly after she was hit by three Japanese bombs on the same day, what would cause the sinking of the aircraft carrier on 7 June 1942, in which 141 men were killed. Colorized U.S. Navy photo by Cassowary Colorizations / CC BY 2.0 DEED.

The Battle of Midway was a pivotal naval battle during World War II fought in the Pacific Theater. It owes much of its outcome to U.S. Navy intelligence. At the heart of the American victory at Midway was the successful breaking of Japanese codes by U.S. Navy intelligence. The team responsible for this critical task was known as Station HYPO, led by Lieutenant Commander Joseph “Joe” Rochefort. Their efforts allowed them to intercept and decipher Japanese communications, revealing the Japanese Navy’s plans to attack Midway Atoll.

The Battle of Midway took place from June 3, 1942, to June 6, 1942. The battle, which primarily involved aircraft carriers, resulted in the U.S. destroying Japan’s first-line carrier strength and most of its best-trained naval pilots. This victory effectively ended Japan’s ability to prosecute an offensive war in the Pacific..

Midway Atoll, located in the Pacific Ocean, served as the battleground for this historic clash. The American forces, led by Admiral Chester W. Nimitz, successfully thwarted further Japanese invasion attempts in the Pacific, marking a turning point in the war.

The extent of Japanese military expansion in the Pacific, April 1942. Wikimedia / CC BY-SA 3.0 DEED.

Station HYPO

Station HYPO, also known as Fleet Radio Unit Pacific (FRUPAC), was the U.S. Navy’s signals monitoring and cryptographic intelligence unit based in Hawaii.

In the summer of 1941, a small team of intelligence analysts, linguists, and code-breakers established the newly renamed Combat Intelligence Unit in the basement of Building 1 at Pearl Harbor. This nondescript location would become Station HYPO.The name “HYPO” originated from the phonetic code for the letter “H,” representing Heʻeia, the location of a radio tower in Hawaii. Hypo’s primary responsibility was to work on Japanese Navy systems. They focused on intercepting and deciphering Japanese naval communications.

Before the attack on Pearl Harbor, the amount of available Japanese Navy traffic was limited. However, HYPO’s collaboration with crypto groups in Melbourne, Hong Kong, and Batavia (now Jakarta) allowed them to share efforts and insights. Their breakthrough came from successfully breaking the Japanese naval code known as JN-25.

HYPO had little doubt about the significance of the code “AF” in intercepted Japanese messages. It referred to the U.S. naval and air base on Midway Atoll, a critical location in the central Pacific.

LCDR Joseph J. Rochefort led and handpicked many of the key codebreakers at HYPO. US. Navy photo.

Intelligence Collection and Analysis

In a clever ruse, the American garrison at Midway sent a fake message “in the clear” (on open channels) about broken water evaporator units on the island. Almost immediately, Japanese transmissions confirmed the water shortage and the need to bring extra water for the operation. This confirmed that Midway was the Japanese objective.

On May 27th, Lieutenant Commander Edwin “Eddie” Layton presented the fruits of HYPO’s work at the Pacific Fleet staff conference. Based on their intelligence, the U.S. planned to ambush the Japanese force near Midway. Layton’s assessment allowed Admiral Chester Nimitz to take a “calculated risk” by committing three precious aircraft carriers to the battle. The foreknowledge provided by this intelligence justified the presence of the USS Yorktown (CV-5), which had been damaged at the Battle of Coral Sea but was rushed back into action after frantic repairs at Pearl Harbor.

Turning the Tide

Between June 4 and 7, U.S. carrier-based aircraft from the Enterprise, Yorktown, and Hornet ambushed and sank the Imperial Japanese Navy’s carrier force. This victory not only avenged Pearl Harbor but also marked a turning point in the war. The Battle of Midway demonstrated how effective intelligence, when skillfully applied, could alter the course of history.

Devastators of VT-6 aboard USS Enterprise being prepared for takeoff during the Battle of Midway. U.S. Navy photo.

The Japanese reaction to their defeat at the Battle of Midway was significant and multifaceted. As a result of the U.S. victory, Japan abandoned its plan to expand its reach in the Pacific. The defeat forced them onto the defensive for the remainder of World War II. The battle marked a turning point, injecting U.S. forces with confidence and draining Japanese morale.

The defeat at Midway altered the course of the war in the Pacific. Japan’s momentum was halted, and they were no longer able to dictate the terms of engagement. The balance of power shifted in favor of the Allies.

Interestingly, shortly after their defeat at Midway in April 1942, the Japanese began peace feelers. They realized that their initial success at Pearl Harbor had been followed by setbacks, and Midway was a critical turning point. The setback guaranteed their eventual surrender.

In this still from the 1942 U.S. Navy documentary film “The Battle of Midway”, shot by the legendary John Ford, soldiers and civilians inspect the wreckage of a plane while black smoke billows in the distance.  Ford shot much of the footage for this film himself and was wounded by shrapnel during the fighting.


In summary, U.S. intelligence played a crucial role in deciphering codes, predicting Japanese movements, and enabling the successful defense of Midway. It remains an enduring lesson on the importance of intelligence in warfare.


Naval History and Heritage Command

Office of Naval Intelligence

By Eugene Nielsen

Eugene Nielsen provides intelligence and security consulting services. He has a bachelor's degree in political science from the University of California. His byline has appeared in numerous national and international journals and magazines.

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