Even before the Japanese attack on Pearl Harbor on December 7, 1941, the U.S. Navy had realized its intelligence operation was woefully inadequate. The Navy needed code-breakers, what are more formally known as cryptanalysts, and it began recruiting them … at the elite women’s colleges of the Northeast.

After the attack, the need became even more pressing, and the U.S. Army jumped on board. Between them, the two services—which were so competitive one wonders at times who they considered the true enemy in World War II—recruited more than 10,000 women to work at its top-secret code-breaking operations in Washington, D.C. Their story is told for the first time by journalist Liza Mundy in Code Girls: The Untold Story of the American Women Code Breakers of World War II (Hatchette Books). Of the 20,000 code-breakers in both services, more than half were women.

Mundy’s book is a welcome addition to the ranks of recent books revealing the crucial supportive work of women in war and peace, including Denise Kiernan’s The Girls of Atomic City, about the women who worked on the Manhattan Project in Oak Ridge, Tennessee, and Margot Lee Shetterly’s Hidden Numbers, about the black women mathematicians at NASA.

While the Navy recruited at Wellesley and the other “fancy women’s colleges of the Northeastern Seaboard,” Mundy writes, the Army “sent recruiters to teaching colleges, far humbler institutions, throughout the South and Midwest.” For example, Winthrop College in South Carolina sent dozens of its graduates who had taken classes in cryptanalysis without knowing the classes were being used by the military to scout talent.

Both services were looking for women with high aptitude in science, math, languages, and music, all skills that suited them to the intense mind-work of analyzing strings of numbers for patterns. They also wanted women of “character and loyalty and grit,” and women who could keep their mouths shut about what they were doing. They all signed secrecy oaths and couldn’t discuss their work even with other women with whom they worked.

Mundy interviewed more than twenty of these “code girls” for her book. Some of them came to Washington as civilians working for the Army at a former women’s college in Arlington, Virginia called Arlington Hall, which it converted into its code-breaking nerve center. Others worked as civilians for the Navy or, more often, joined the WAVES, an acronym for Women Accepted for Volunteer Emergency Service. The Navy took over another women’s college, Mount Vernon Seminary, in Northwest Washington, which it re-named the Navy Communications Annex.

Many of these women, especially at Arlington Hall, were small-town girls who had never ventured far from home. Many had brothers or boyfriends serving in the armed forces, and wanted to do their part to bring them home safely. They faced long hours, crowded living conditions, condescension and sexual harassment. The nature of the work drove some of its most gifted practitioners to nervous breakdowns.

Mundy writes: “It was boring, tedious work, except when it wasn’t.” When a cryptanalyst was able to decipher a crucial message, she might be providing the Navy with the intelligence it needed to sink a Japanese convoy. The work done prior to D-Day helped divert the Germans’ attention from Normandy—and the analysis of German messages, thanks to the breaking of the Enigma machine by British and American intelligence, let them know the Germans had been fooled.

Most of the code girls in the book said working in Washington during the war was the most exciting time of their lives. Mundy’s lively descriptions of their experiences add levity to the pages of the book that explain the work they did. (Readers who aren’t mathematically inclined may want to skip those pages. Denise Kiernan’s approach, alternating sections on living at Oak Ridge with the science of the atomic bomb, was a more palatable one for this reader.)

The code girls who are living today, like the boys who went off to war, are in their late 80s and 90s. Liza Mundy’s book is a fitting tribute to these unsung heroines, whose work saved many American lives and undoubtedly shortened the war.