When she was asked to spy for the French army, Marthe Cohn recalled decades later, she didn’t hesitate before saying yes. It was late 1944, months before the end of World War II in Europe. She had already lost her fiancé, a member of the French resistance who had been tortured and shot by the Germans outside Paris. One of her younger sisters, Stéphanie, had also been killed — arrested and deported to Auschwitz while trying to help people escape to unoccupied France.

Ms. Cohn, a French Jew, had managed to survive the Nazi occupation with help from false identity papers provided by a friend. She was also aided by her blond-haired, blue-eyed appearance and her fluency in German, which she learned as a child in Metz, a French city not far from the border. After joining military intelligence, she was able to cross enemy lines and spend a month in Germany, where she passed as a German nurse while making small talk with SS officers and Nazi soldiers, gathering information about troop movements.

Her spying earned her France’s Croix de Guerre and was credited with saving the lives of Allied troops pressing in on the Reich. And more than 50 years later, after French officials took a fresh look at her military record, she was awarded another prestigious award, the Médaille Militaire, and was named a knight in the Legion of Honor, the country’s highest order of merit.

By then, Ms. Cohn had only just begun to discuss her brief career as a spy. Her husband, an American, had learned about her espionage exploits only after they were married. For years, even her children had no idea that Ms. Cohn — a petite but energetic woman who stood no more than 4-foot-11 - once crawled across the border on her hands and knees, hiding from German sentries while bringing intelligence back to the French.

“I just thought nobody would believe me,” she told the Los Angeles Times in 2005, explaining her years of silence. “Spies are usually tall and good-looking. I am a very unlikely spy.”

Ms. Cohn, who died May 21 at the age of 105, spent the past quarter-century sharing her story at schools and community centers across Europe and the United States, where she worked as a nurse after the war. In her final years, she served as a memory keeper for the Holocaust and the French resistance, sharing her story in a well-received 2002 memoir, “Behind Enemy Lines: The True Story of a Jewish Spy in Nazi Germany,” and in a 2019 documentary, “Chichinette: The Accidental Spy.”

“I will bear witness,” she often told audiences, “until my last breath.”

The fourth of seven children, she was born Marthe Hoffnung in Metz on April 13, 1920. Her family was Orthodox — her maternal grandfather was a rabbi — and her parents ran a small business framing and enlarging photos.

As a teenager, Ms. Cohn occasionally got into fistfights at school, brawling with Catholic classmates who made antisemitic comments about Prime Minister Léon Blum, who was Jewish. She said she inherited some of her scrappiness from her father, who once removed his belt and went after teenagers who were throwing stones at the family as they left the local synagogue.

After Kristallnacht, the Nazis’ 1938 pogrom against Jews, Ms. Cohn’s parents began taking in Jewish refugees from Germany, looking after penniless families that needed a place to stay for a few days as they sought a new home in France or elsewhere. Ms. Cohn recalled in her memoir that while she was horrified by the pogrom, “never for one moment did I think that the same thing would happen to us. Not in France. … I believed in human nature. I still had confidence that good would prevail.”

When World War II broke out in 1939, the family moved across the country to Poitiers, far from the German border. They remained there during the 1940 invasion and subsequent occupation, living for a time under few restrictions. Ms. Cohn said that she even worked at city hall with German officials who, admiring her accent and skill with the language, invited her to move to Germany for work, not realizing she was Jewish.

Gradually, the situation deteriorated. Nazi leaders closed Jewish businesses and mandated that residents wear a yellow star in public. Ms. Cohn was approached on the street one day by one of her colleagues from city hall, who offered to provide her and her family with identity papers that were not stamped with the word “Jew.” The documents would let them travel freely to unoccupied France.

“When I asked him how much it would cost, he started crying and he said, ‘I do not want to be paid. I do this to save you,’” she recalled in an interview with the Jewish Ledger, a Connecticut newspaper. “He gave me all the identity cards.”

Ms. Cohn’s sister Stéphanie was arrested before the family could leave. But Ms. Cohn, her parents and several of her siblings were able to flee and survive, with help at times from one of Ms. Cohn’s brothers who worked in the resistance. She spent part of the war in Marseille, studying to become a nurse, and enlisted in the French army in 1944 after the liberation of Paris.

“At first,” she recalled, “they looked at my size and said, ‘Little girl, go back to your mother. You don’t belong in the army.’”

Ms. Cohn proved persistent — “I’m going to stay,” she said she told the officers — and was allowed to enlist as an aid worker, tasked with visiting soldiers near the front and asking what they needed. At one point, she was assigned to answer calls for a colonel who needed to step out for lunch. He apologized, telling Ms. Cohn that she would have nothing to read to pass the time, as he only had German-language books in his office.

“I said, ‘That’s OK, I can read German,’” she told the Los Angeles Times in 2000. “It’s as simple as that, how your life can change.”

The army was looking for German-language speakers who could work as spies. Ms. Cohn underwent a brief training — she was so inquisitive, she said, that her colleagues nicknamed her Chichinette, French for “little pain in the neck” — and was sent to Switzerland, where she tried more than a dozen times to cross the border into Germany. She eventually succeeded, crossing a field without being spotted, carrying only a small suitcase and a picture of a German prisoner of war whom she claimed was her fiancé.

Ms. Cohn used the photo to win the trust of German soldiers, asking whether they had seen her lover on the battlefield.

During one encounter, she claimed to be terrified about the prospect of an Allied invasion. “They told me not to worry,” she said in the Times interview. “And then they told me in precisely which section of the Black Forest the German army was waiting for the Allies.”

Ms. Cohn hurried back to the border to share her discovery with the French. She also revealed that German troops near Freiburg were withdrawing from the Siegfried Line, a long-fortified defensive position.

After the war ended, she served as an army nurse in Vietnam, then part of French Indochina. She also continued her nursing studies in Geneva, where she met Major L. Cohn, an American medical student who had served on a Navy minesweeper during World War II. They married in 1958, moved to the United States and later worked together in Los Angeles, her husband as an anesthesiologist and Ms. Cohn as a nurse.

Her husband, who survives her, said Ms. Cohn died at home in Rancho Palos Verdes, California. He did not cite a specific cause. Survivors also include their two sons, Stephan and Remi Cohn, and a granddaughter.

In her public appearances, Ms. Cohn sought to draw lessons from the Holocaust, urging audiences to have sympathy for migrants who — like many Europeans Jews in the 1930s and ’40s — struggle to find refuge in the United States and other countries. Asked in the documentary what message she had for people today, she replied, “Be engaged. And don’t accept any order that your conscience could not approve.”