A Woman of No Importance. By Sonia Purnell.Viking; 368 pages; $28. Virago; £20.

A S TALES OF wartime derring-do go, it would be hard to beat that of Virginia Hall, a young, one-legged American woman who, in the Gestapo’s view, became the Allies’ most dangerous spy. She did more than anyone else to forge the disparate, rivalrous groups of the French Resistance into effective military units that by 1944 could play a part in liberating their country. As Sonia Purnell shows in her new biography, Hall’s bravery was of the cool, calculating, unflagging kind that is peculiarly required of the special agent operating for years in enemy-occupied territory, in constant danger of betrayal or of making the one wrong false step that would result in exposure, capture, torture and death.

From the outset, she seemed to have known she was different. Born in Baltimore in 1906 to conventional, upper-middle class parents, she insisted on going to university (Radcliffe College, the bluestocking offshoot from Harvard) and completing her studies in French, German and Italian in Europe. Her ambition to join the State Department was thwarted first by bureaucratic misogyny and then by a hunting accident in Turkey when she was 27, which led to the amputation of her leg and the fitting of a prosthesis she named “Cuthbert”.

Unbowed and determined to relay the horrors of fascism to readers at home, she became a stringer in Europe for several American newspapers. By the summer of 1940, as German Panzers rolled through France, she had found new work driving wounded soldiers from the collapsing French army to hospitals in Paris. It was then that she had an idea. As the citizen of a neutral country, she could exploit her relative freedom to move around by becoming an undercover agent for Britain’s nascent Special Operations Executive ( SOE ), which sent her back to France in 1941.

Although continually patronised and underestimated, Hall quickly adapted to the secret life, basing herself in Lyon, deep in collaborationist Vichy France, and exploiting her cover as a journalist. Her bosses in London soon saw that she had talents they could use. She was an able recruiter of intelligence assets, including a courageous brothel madam, several prostitutes and a VD doctor. At a time when the Resistance barely existed, she found and trained saboteurs and developed escape routes for downed British pilots and brave but bungling agents sent from London. She even organised spectacular jail breaks when colleagues were captured by the Germans or the French police. When other agents were slapdash and guilty of lethal security breaches, she somehow kept the show on the road, even as the personal risks to her intensified. Klaus Barbie, the psychopathic “Butcher of Lyon”, became obsessed with killing the “limping lady”.

Eventually Hall’s luck ran out. Betrayed by a clever and vile double agent, Abbé Alesch, her network shattered (many of her associates were tortured and sent to death camps), her own cover blown, she escaped from France by crossing the Pyrenees in midwinter on foot, her stump oozing blood as Cuthbert fell apart. Once back in London, Hall resolved to return to France to help prepare the ground for D-Day. When SOE refused to send her back, deeming the risks too high, she persuaded the British outfit’s fledgling American counterpart, the Organisation of Strategic Services ( OSS ), to take her on. Operating in the Haute-Loire region in the guise of a milkmaid—the Madonna of the Mountains, as Hall was dubbed by her Resistance recruits—she shaped her men into an insurgent force capable of liberating the region with little need of external help. Intelligence provided by Hall on the disposition of the German Seventh Army led the Americans to trap and destroy it in the Falaise Gap, resulting in the decisive breakthrough in the Battle of Normandy.