6 American Codebreakers Who Helped Change the Course of WWII
World War II wasn't won with just flashy tanks, large ground assaults, and new weapons. Hidden behind the spotlight were American codebreakers, slowly churning through enemy messages, decoding them into useful intelligence to help the Allied powers win. Their codebreaking helped with attacks, troops, enemy movements, and understanding the ins and outs of Japanese and German plots. Using machines, pens, and paper, these American codebreakers were more crucial to the war than you might think.
1. Joseph Rochefort — Helped the U.S. win the Battle of Midway
Joseph Rochefort took part in the intelligence operation surrounding the Battle of Midway in June 1942. At Station HYPO in Hawaii, he led a team working to decipher Japanese naval communications. According to the National Security Agency , his unit produced information on Japanese forces and intentions, including information that proved vital before Midway. Rochefort's team determined that Midway was the Japanese target, allowing Admiral Chester Nimitz to prepare an ambush. The resulting American victory became a major turning point in the Pacific War for American codebreakers.
2. Agnes Meyer Driscoll — Laid the groundwork for cracking Japanese naval codes
Long before Midway, Agnes Meyer Driscoll was already tackling Japanese naval codes for the American Navy. She was a pioneer whose work helped create the foundation later cryptanalysts built upon. The Naval History and Heritage Command notes that Driscoll trained important Navy cryptanalysts, including Joseph Rochefort and Thomas Dyer. Impressively, much of Driscoll's work was done manually, using techniques that required patience and extraordinary mathematical skill. Her efforts gave the Navy a stronger starting point against the enemy when the Pacific war began.
3. Thomas H. Dyer — Turned codebreaking into a faster, more practical process
Thomas H. Dyer brought something new to cryptanalysis: machines. While working for the Navy, he developed procedures for using IBM tabulating equipment to sort through the enormous number of possibilities involved in breaking codes. Articles from the NSA show why he earned the nickname "father of machine cryptanalysis." At Station HYPO, Dyer became a leading figure in breaking Japanese naval communications. By May 1942, their intelligence was detailed enough to help Nimitz anticipate the Japanese attack on Midway. Dyer also later contributed to the operation that led to the shootdown of Admiral Isoroku Yamamoto.
4. Elizebeth Smith Friedman — Saved thousands of troops from Nazi intelligence
A single decoded message could be the turning point between life and death. Elizebeth Smith Friedman proved this repeatedly while tracking Nazi spies in South America during World War II. An article from PBS shared that her team intercepted German communications revealing the movements of Allied ships. One particular example involved the troopship RMS Queen Mary, which carried 8,398 American servicemen. Her team eventually broke thousands of messages and helped disrupt Nazi espionage efforts across the Western Hemisphere.
5. William F. Friedman — Cracked Japan's "Purple" diplomatic cipher
William F. Friedman was already a major figure in American cryptology when Japan introduced its sophisticated Purple cipher. Friedman headed the U.S. Army team that broke the system in 1940, gaining access to highly sensitive Japanese diplomatic communications. According to the NSA , General George Marshall later credited Friedman's team with saving American lives and helping bring the war to an earlier end. The decrypts, known as MAGIC, gave American officials valuable insight into Japanese policy and its relationship with Germany and Italy.
6. Frank B. Rowlett — Helped unlock Purple and protected American communications
Frank B. Rowlett was one of William Friedman's earliest recruits, and his work became essential to solving Japan's Purple cipher. NSA states that Rowlett played a major role in the breakthrough, leading the day-to-day effort while Friedman directed the overall project. During World War II, Rowlett also worked with the Navy on SIGABA, an American cipher machine that the Axis powers never successfully cracked. It gave U.S. forces a secure way to handle sensitive military communications .
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