The recently published Code Girls by Liza Mundy tells the extraordinary story of the 11,000 women who worked for the U.S. Army and Navy as code breakers during World War II.

They were recruited from the Seven Sisters and other colleges throughout New England by the Navy and from colleges and school-teaching positions throughout the rest of the U.S. by the Army. All of the initial ones came in as civilian workers, though quite a few of both those initial recruits and the later arrivals became WACs (Women’s Army Corps) and WAVES (Women Accepted for Volunteer Emergency Service), both commissioned officers and enlisted women, following the creation of the women’s corps. Most served in Washington, D.C. or Dayton, Ohio, though some in the Army served in the European theater. Much credit is due to those men in military leadership who recognized the important role that very bright women could play in this key wartime effort and encouraged their use in this way.

The recruited women were selected through rigorous testing in mathematics, puzzle-and-problem solving, and linguistics. All were sworn so intensely to secrecy that they didn’t discuss their work even after the war ended, and their accomplishments went publicly unheralded until recently notwithstanding how critical their work was. Some of the women went on after the war to serve in the newly created NSA, and a few became leaders in America’s code-breaking and other intelligence work through much of the rest of the 20th Century.

The war began with stunning U.S. losses at Pearl Harbor and in the North Atlantic. In the perilous Atlantic crossing, enormous numbers of Allied vessels were being sunk by U-boat packs appearing at unpredictable times and locations. In fact, vessels were being lost faster even than the amazing rate of U.S. production (a Liberty Ship a day at the peak) could replace them. In short, the need for both strategic and tactical intelligence was enormous, yet the major encoding techniques and devices employed by both the Germans and the Japanese were devastatingly difficult to break.

Much has been made of the successful British efforts to break Germany’s Enigma coding machinery under Alan Turing’s leadership at Bletchley Park. This book illuminates the key role American women code breakers played in that process and in the design and large-scale production of code-breaking machinery able, with much human ingenuity guiding them, to break Enigma-transmitted messages almost in real time. The result was the retrieval of U-boat messages in time to enable the rerouting of whole convoys and the destruction of the U-boats that tried nonetheless to intercept them. In fact, by the time of the build-up for D-Day, much of Germany’s U-boat fleet had been obliterated.

At the same time as the Enigma work was moving forward, other women code breakers were working on breaking the Japanese Army and Navy codes. Their naval-code work led to major U.S. naval victories at Midway and thereafter. Likewise, the breaking of Japanese Army codes allowed General MacArthur’s campaign through the South Pacific and up to the Philippines to proceed with detailed advance knowledge of Japanese intentions, order of battle, and troop dispositions and casualties, while the breaking of the Japanese naval codes, including ones for its commercial fleet done by a group of African-American women, allowed the Navy and the Army Air Corps to destroy most of Japan’s military and commercial fleet. The result was denial to Japanese troops in their Pacific island strongholds of food (to the point of starvation), medicine, and reinforcements. Meanwhile, the breaking of Japanese diplomatic codes allowed an inside look into Axis strategic thinking and troop dispositions, including along the English Channel pre D-Day, as Japanese ambassadorial staff reported back to their superiors in Japan on their high-level discussions with German leadership and their detailed observations on tours of German fortifications.

The complex work these women, who formed the bulk of our nation’s code breakers, did was simply astounding, yet when the war ended, they were largely displaced by men and urged to turn exclusively to “family duties,” as were so many other successful women workers of that era. As the MeToo movement and Women’s Marches progress, I’m reminded of those extraordinary women whose efforts revolutionized code breaking and, at a minimum, shortened the war significantly — and even perhaps helped to determine its outcome. Three-quarters of a century later, we are still struggling with the proper treatment of women in the workforce. Now, though, women are taking a leading role in assuring that the future will be different from the “use them and lose them” attitudes of the past.