The women codebreakers of World War II

How more than 10,000 women worked with the U.S. military to help end the war

One chilly afternoon in November 1941, Ann White found an odd letter in her mailbox from an astronomy professor who wanted to meet. White, who was studying German at Wellesley College in Massachusetts, soon discovered that the professor had just two questions: Do you like crossword puzzles? And, are you engaged to be married?

These same two questions were asked of women across the United States. If the answer to the first was yes and the second no, the women were offered the chance to train for a secret career: breaking codes for the United States Army or Navy.

Just two years before, Germany—ruled by the Nazi party leader Adolf Hitler—had invaded Poland, marking the start of World War II. Tensions were still high following the end of World War I in 1918, and Hitler's invasion of Poland and other European countries led to a conflict that spread around the world.

Countries generally split into sides: the Axis powers (whose major players included Germany, Italy, and Japan) and the Allies (which included Britain, France, Russia, China, and the United States). The United States entered the war on December 7, 1941, after the Japanese bombed the Pearl Harbor Naval Base on the Hawaiian island of Oahu.

Many countries developed codes to send secret messages about things like upcoming battles or troop movements. Deciphering, or breaking, these codes let the Allies listen in on their enemies’ plans and learn about battles before they even happened.

As the United States prepared to send men into the war, the country needed people to help with codebreaking. And women, including Ann White, rose to the task. According to the book Code Girls by Liza Mundy, more than 10,000 women would serve for the U.S. Army and Navy, making up more than half the U.S. codebreakers. Their hardwon efforts were key to winning the war.

‘Loose lips sink ships’

People have been sending and breaking secret messages for centuries. For example, George Washington led a group of spies who communicated with codes during the Revolutionary War. The invention of the radio allowed militaries during World War I to send secret messages faster than ever before—and created more chances for others to listen in.

In 1917, the U.S. Army created its first code and cipher bureau (a cipher is another way to make a message secret), which was later replaced with the Signal Intelligence Service. Soon after, the Navy created its own group that tracked communications between enemy ships and submarines. (It was officially called OP-20-G, but people who knew about it just called it the "Research Desk.") Although important, these codebreaking units were fairly small.

As the military realized they might soon have to enter World War II, they slowly started to search for more codebreakers like Ann White. But when more than 300 Japanese planes swooped into the skies above Honolulu's Pearl Harbor in a surprise attack, the United States knew they had to do more to protect the country. That’s when the military began a big push to find codebreakers to make sure the country wouldn't be taken by surprise again. And with so many men headed to the battlefields, women were needed to fill these roles.

At the time, most women did not have careers and instead got married and took care of their home, husband, and children. In fact, in the 1940s, only about four percent of U.S. women had completed four years of college.

But the sudden need for codebreakers offered a new opportunity for college-educated women who claimed they weren't engaged or married. (Employers back then figured that once a woman was married, she wouldn’t have a paying job.) Many women at U.S. colleges received letters similar to Ann White's. Others answered newspaper ads or signed up after meeting with military recruiters who traveled across the country looking for codebreakers.

The women all went through difficult training courses to learn about deciphering coded messages. They promised to tell no one—even their families—what they were doing, in case enemy countries were listening. As they were often reminded, "Loose lips sink ships."

Searching for patterns

The women codebreakers served many roles. They listened for coded radio messages from other countries. They collected things like the names of enemy ships and commanders that could be in a coded message—and therefore could help break it. They looked for patterns—for instance, where and when messages were sent could be clues about troop movements. They even tested American codes to make sure enemies couldn’t break them.

Teams that were often mostly women also broke codes and ciphers themselves, sometimes using complex technology and equipment to help. The codebreakers studied long strings of letters and numbers, looking for patterns that might reveal a message hidden in sentences that looked like nonsense.

The work required a strong understanding of math since many ciphers replace letters with numbers and then hide the original message with equations. The codebreakers also needed a good memory, patience, and persistence. The women learned tricks, such as looking for short phrases that sometimes marked the start of the coded notes, like "begin message here." This would help them pry open the rest of the code—but the work was never easy.

Throughout the war the codes and ciphers changed, growing more and more complex. So each code needed to be cracked again and again. But codes weren't the only challenges the women faced.

Even though the women codebreakers were doing the same work as men, they were paid less. Some of the military men treated the women poorly because they didn’t think the women were important. According to Mundy’s book, one group of sailors ordered Navy women codebreakers to wash windows until a female officer stopped the task.

The codebreakers took turns working around the clock, always aware their failures could mean that American troops might die. "Everyone we knew and loved was in this war," White recalled in Mundy’s book.

Ending the war

The codebreakers’ work was crucial to winning the war—and women were an important part of every team.

Genevieve Grotjan identified the key pattern that cracked the "Purple" cipher used to send messages to high-ranking Japanese government officials, which provided some of the most helpful information about their plans. Ann Caracristi helped crack a code that revealed the locations of Japanese supply ships so the American troops could sink them.

Frances Steen was one of many women codebreakers who deciphered messages that helped the United States kill the Japanese Admiral Isoroku Yamamoto, who planned the attack on Pearl Harbor. And Virginia D. Aderholt decoded the intercepted message from the Japanese that they were about to surrender, which ended the war.

Men often took credit for the women’s accomplishments. For instance, J. Edgar Hoover, who was the first director of the Federal Bureau of Investigations, claimed that his group broke up a Nazi spy ring in South America. But actually, that win belonged to the codebreaker Elizabeth Smith Friedman, who helped decipher secret enemy messages during both world wars and is often called America's first female codebreaker.

Without the women codebreakers, some experts believe that the war would’ve lasted two years longer than it did. But many were kicked out of the military after the war ended, and decades passed before they were recognized for their achievements. The women had promised to keep their work secret, and they stayed silent for years.

During this time, many people thought that only men could be geniuses while women were more fit for boring work that didn't take much thought. Although it took decades for the codebreakers' story to be told, their successes show just how brilliant women can be—and how much can be achieved when everyone has a chance to shine.