Code Girls

Code Girls tells the largely hidden story of the more than eleven thousand American women who served as code breakers during World War II. The book recovers a chapter of wartime history that the women themselves were sworn to keep secret, in many cases for the rest of their lives.

The book opens with the Japanese attack on Pearl Harbor on December 7, 1941, when nearly two hundred fighters and bombers struck the US Pacific Fleet in Hawaii, sinking or damaging multiple battleships and killing more than two thousand men. American leaders recognized that their intelligence operations were dangerously inadequate and that a first-rate code-breaking effort was essential. Even before the attack, the US Navy had begun sending secret letters to top female students at elite women's colleges, including Wellesley, Bryn Mawr, Vassar, and Goucher, inviting them to study cryptanalysis, the science of breaking secret codes. The Army, meanwhile, recruited women from teaching colleges and among schoolteachers in small towns across the country. Of approximately twenty thousand total American code breakers during the war, roughly eleven thousand were women. The Axis powers did not employ women in comparable roles, giving the Allies a significant workforce advantage.

The book traces the origins of American code breaking to World War I, highlighting the contributions of the married couple William and Elizebeth Friedman and of Agnes Meyer Driscoll, a former math teacher who enlisted in the naval reserves in 1917 and went on to train nearly all the male code breakers who became famous in World War II. By September 1940, William Friedman's small Army team at the Signal Intelligence Service was struggling to crack the Purple cipher machine, a device used by Japanese diplomats that no Western agency had been able to break. The team included Genevieve Grotjan, a twenty-seven-year-old from Buffalo, New York, who had aspired to teach college math but could not find a university willing to hire a woman. On September 20, 1940, Grotjan identified repeating patterns across worksheets that confirmed how the machine's switching mechanism worked. Her breakthrough allowed the Americans to build a functioning replica. The intelligence it produced, known as "Magic," gave the Allies access to Japanese diplomatic communications from capitals across Europe.

The first six months after Pearl Harbor brought devastating losses. Japan captured territory from Guam to Singapore, while German U-boats sank Allied ships in the Atlantic faster than they could be replaced. Into this atmosphere, the first wave of seventy-four college-educated women arrived at the Navy's Washington headquarters in summer 1942. Margaret Gilman, a Bryn Mawr graduate, was assigned to attack the German Navy's new four-rotor Enigma cipher, called Shark, using hand methods. Ann White, a Wellesley graduate, translated broken Enigma messages and confronted the cost of failure when her team could not locate U-boats threatening an Allied convoy near Greenland. Most of the ships were lost.

A turning point came at the Battle of Midway in June 1942. Code breakers working the Japanese naval fleet code JN-25, a complex system that Agnes Driscoll had spent years diagnosing, identified a massive Japanese operation. When they devised a trick to confirm that Midway was the target, Admiral Chester Nimitz reinforced the island. The four-day battle cost Japan four aircraft carriers and marked the end of Japanese expansion in the Pacific.

As the war intensified, women took on ever-larger roles. The creation of the WAVES (Women Accepted for Volunteer Emergency Service) in July 1942 formalized women's naval service, and enlisted women who tested high for intelligence were quietly funneled into code-breaking work. The Navy relocated its operation to the former Mount Vernon Seminary in Washington, DC, renamed the Naval Communications Annex, where women eventually made up 80 percent of the workforce. At Arlington Hall, the Army's facility in Virginia, women ran top units in an informal atmosphere where rank mattered less than knowledge. Arlington Hall also maintained a segregated African American code-breaking unit that monitored commercial communications, though the unit remained unknown to most white workers and uncelebrated after the war.

At Arlington Hall, the Japanese Army codes remained unbroken until early 1943, when Ann Caracristi, a twenty-three-year-old English major from Russell Sage College, and Wilma Berryman, a trained high school math teacher who was one of William Friedman's early hires, cracked the address code prefacing every Japanese Army message. Using captured message templates, cribs (predictable plaintext clues used to match code groups) from Navy systems, and Berryman's remarkable memory, the two women achieved Arlington Hall's first break into any Japanese Army system. This success was followed in April 1943 by the breaking of the 2468 water-transport code, used to route Japanese supply ships. An elite team working through the night discovered a layered encryption in which the message itself was used to further encipher the indicators, the code groups that signaled how a message had been encrypted. The first decoded message revealed four supply ships in Wewak Harbor, New Guinea; the US Navy sank all four. The 2468 breakthrough opened floodgates of intelligence and drove the Army's intensified recruitment, including that of Dot Braden, a twenty-three-year-old former schoolteacher from Virginia.

The book follows Dot from a miserable year teaching at a rural high school for $900 to her arrival at Arlington Hall in October 1943. Assigned to the 2468 code, Dot memorized four-digit code groups and ran urgent messages to the overlappers' unit, where colleagues grouped messages encoded with the same additives—numbers added to code groups to further disguise their meaning. She formed a lifelong friendship with Ruth "Crow" Weston, a shy math major from Mississippi; the two shared an apartment but never discussed their work. Their efforts contributed to the near-total destruction of the Japanese merchant fleet, cutting Japan off from overseas supplies and contributing to widespread starvation among its troops.

Meanwhile, the Navy secretly sent WAVES to Dayton, Ohio, to wire commutator wheels for American bombe machines designed to break the four-rotor Enigma. Several hundred women returned to Washington to operate 120 bombes around the clock, forming a virtually all-female code-breaking chain. By April 1944, the average delay in breaking the daily U-boat cipher key had dropped to about twelve hours, transforming the Battle of the Atlantic.

Code-breaking intelligence also shaped major operations. In April 1943, intercepted messages revealed Admiral Yamamoto's travel itinerary; American fighters shot down his bomber over Bougainville. For D-Day, the Purple machine deciphered a report from Japan's ambassador describing German coastal defenses, helping commanders choose Normandy for the invasion. Women at Arlington Hall generated fake radio traffic to convince Germany that the main assault would target the Pas de Calais. On the night of June 6, 1944, Ann White's team broke a German message announcing the invasion. In the aftermath, bombe operator Jimmie Lee Hutchison Powers learned that her husband, a glider pilot, had been killed; her request for leave was denied because too many operators were receiving similar telegrams.

The war's final stages brought savage losses on both fronts. Dot's brother Teedy, who had enlisted at eighteen, survived the Battle of the Bulge after his armored car was hit by a German rocket. Elizabeth Bigelow, a code breaker recruited from Vassar, lost her brother Jack when a kamikaze struck the escort carrier Suwannee at the Battle of Leyte Gulf. Fran Steen, a Navy watch officer at the Naval Communications Annex, broke a message revealing that her brother Egil's destroyer was targeted for a kamikaze raid; she alerted the Navy, and Egil survived. On August 14, 1945, after atomic bombs were dropped on Hiroshima and Nagasaki, Arlington Hall intercepted Japan's surrender message, transmitted in the JAH cipher, a Japanese diplomatic encryption system. Virginia Dare Aderholdt, an Arlington Hall code breaker who specialized in Japanese diplomatic traffic, decoded it before the intended Japanese recipients received it. Everyone at Arlington Hall took a vow of silence until President Truman's official announcement at seven p.m., when Washington erupted in celebration.

After the war, most women returned to civilian life and kept their secret for decades. Dot married Jim Bruce in December 1945 and settled in Richmond, Virginia. A few women stayed in the field: Ann Caracristi rose to become the first female deputy director of the National Security Agency. The book notes that the postwar Joint Committee investigating Pearl Harbor praised Army and Navy intelligence as having "contributed enormously to the defeat of the enemy, greatly shortening the war, and saving many thousands of lives" (16), but the fact that more than half of these code breakers were women was never mentioned.