It was ingrained in them, of course, to think of war as men’s work, to measure success by what could be overpowered. Which is why their hearts sank when a wispy young woman strolled into the room. Andrée de Jongh had dark, fluffy hair, a pert mouth and high-arching eyebrows. She wore a flowered dress and white ankle socks. She was 24 but looked 18, weighing about 100 pounds. She seemed to take up no space at all. She introduced herself only as Dédée, a nickname. “Our lives,” the Australian announced glumly after she’d gone, “depend on a schoolgirl.” At breakfast the following day, she advised them that they’d begin their journey together by jumping into the quick-moving Somme River and swimming clandestinely across.

Her implausibility was what made her formidable. When the war started, Dédée de Jongh quit her job as a commercial artist and moved into her parents’ house in Brussels. Volunteering with the Belgian Red Cross, she began nursing wounded Allied soldiers. After her country surrendered to the Nazis in May 1940, after British troops were evacuated and the battle shifted to the air, de Jongh turned her attention to the men who had been shot out of the sky.

Nursing was fine wartime work for women, but in the end, not for her. According to the author Derek Shuff, whose 2003 book “Evader” chronicles Jack Newton’s journey out of Belgium, de Jongh found her life to be “tedious.” She craved more. “I was very impatient to do something,” she would later say. A woman couldn’t carry a gun or fly a bomber jet, but she could walk unnoticed, striding down a street in a wool coat and sensible shoes as if on her way to the market or a typist’s job, trailed quietly by two or three wayward soldiers in disguise.

She found them places to stay and led them there. But safe houses were temporary and only marginally safe. She became fixated on getting them home. She pressed friends and relatives to give money, food, shelter, to forge ID papers for escaping soldiers.

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In August 1941, she made a trial run. With the grudging help of a Basque guide, who insisted she at least change out of her skirt and into a pair of borrowed trousers, de Jongh climbed a smugglers’ route over the Pyrenees and into Spain with two Belgians and a Scottish soldier in tow. Arriving at the British consulate in Bilbao, she was greeted with skepticism. You’ve done what?

Within weeks, de Jongh delivered two more Scottish soldiers to officials in San Sebastián. Not long after, she arrived from Brussels with a group of 11 escaping civilians — 10 Belgian men and a portly fleeing Englishwoman in a Panama hat, whom de Jongh had pushed across the Somme River in a rubber tire. Understanding that the return of lost R.A.F. soldiers would boost his country’s sagging morale, the British consul to Spain agreed to finance de Jongh’s network. Intelligence officers code-named her “the Postman.”

To the men who were to travel with her, she gave lessons on invisibility. No. 1, they were to walk at least 15 feet behind her at all times, on the street and in train stations from Brussels to Paris to Bayonne. No. 2, in public they were never to speak — not to her, not to one another, not to anybody. If an airman had a friendly face, one that might invite even an innocent conversation on a train, she armed him with a copy of the French newspaper Le Figaro and instructions to keep it hoisted. To others, she gave oranges to be peeled messily every few hours on a train ride, simply as a means of repelling their neighbors.

Before it was over, Dédée de Jongh would personally escort 118 people to freedom in Spain, and hundreds more would escape using the complex network of safe houses she had set up throughout Belgium and France. She pep-talked countless men over the mountains, including Jack Newton, who, depleted but grateful, was sent to Gibraltar and put on a boat home to his wife. Many of her helpers were ultimately arrested — including her sister, who was sent to a concentration camp, and her father, who was shot by a German firing squad.

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De Jongh herself was caught harboring three airmen in a farmhouse at the foot of the Pyrenees in 1943. She endured 20 interrogations before finally confessing not just to being involved with the famous Comet line but to being its mastermind. Her German captors dismissed the idea outright. “Don’t be ridiculous,” they said.

Sometime later, the Gestapo thought to question her further, but when they went looking among the emaciated pale souls packed into the Ravensbruck women’s concentration camp north of Berlin, they could not figure out which one was Dédée de Jongh.

After the war, she was decorated by King George VI and honored by the American and French governments. In Belgium, she was named a countess. She waved off most of the attention and strived instead for a purposeful sort of invisibility, spending 28 years nursing at a leper colony in the Belgian Congo and at an Ethiopian hospital. Only when her health began to fail did she return to Belgium, where the skies were empty and the farmers’ fields hid nothing but old stories, the safe houses she’d created now just the same as every other.