Edith Cavell, By Diana Souhami

It is surprising to read the biography of someone as famously brave and selfless as Edith Cavell and find no suggestion in her life before the age of 40 that there was anything exceptional about her. Born in 1865, this English vicar's daughter, who gained fame as a nurse during the Great War, did not even embrace the profession until she was 30. Following training and experience, her "reserve" and "self-sufficiency" may have been the cause of her difficulty in gaining a senior position until she was approached in 1907 by a Belgian doctor to open and supervise a School for Nursing in Brussels. Compared with nursing in Britain, which had undergone comprehensive reform as a result of Florence Nightingale's efforts, Belgium was like "going back in time". There was resistance to anyone nursing but nuns ("sometimes dirty and always ignorant," opined Cavell). Respectable mothers no more wanted their daughters to train as professional nurses than to go on the stage.

Given a free hand and the Christian zeal to serve borne of her upbringing, Cavell was at last in her element. Her school (in which student nurses ate, slept and treated patients) soon became a much-copied model of good practice. In the summer of 1914, as she and her nurses watched German soldiers march into the Brussels, Cavell warned her nurses that all wounded soldiers must be treated alike, friend or foe.

The work treating the German wounded was overwhelming: the nurses slept, often fully dressed, on their beds, according to one "sometimes weeping". Soon after, Cavell's work began with what the French termed les enfants perdus – Allied soldiers who got separated from their regiments, isolated in enemy-occupied territory. Many died of untreated wounds or were arrested. Others were fortunate to be taken in by local people and passed along a network of safe houses and, so they hoped, to Holland and freedom.

One afternoon two wounded British soldiers arrived in disguise at the school asking for help. Sister Cavell unhesitatingly took them in, and soon provided not only a "safe house" but allowed the school to become a hub of the escape network. It was a route which was to end in her execution before a firing squad.

Cavell realised that young soldiers were not as amenable to discipline as her probationer nurses. The first large cohort consisted of about 20 British soldiers, concealed in a large attic room. "Edith Cavell told them they could go out in the evening for walks, singly or in pairs, but that they must be back by 9.00... She warned there were many German officers billeted in houses close to the Clinic." On the third night, two had returned by 10pm. The others got drunk, "got into a fight and rolled back at midnight singing 'It's a long way to Tipperary'."

Later visitors were much more careful, as the school became the subject of suspicion and routine searches, often at night. One one occasion, Cavell hastily got an unofficial resident out of bed, made him climb into a barrel in the garden and covered him with apples.

On 4 August 1915 came the inevitable knock at the door. Cavell and many others of the network were arrested simultaneously to prevent them conferring. Securing Cavell's conviction was important. Of all the accused, she was the only one who was English, and "viewed by her prosecutors as a small but vital and tangible incarnation of the enemy".

Now she exhibited a new kind of bravery. As the trial was conducted in German, few of the accused understood much, except when direct questions were translated. They were all feeling the pressure: one seemed "crushed"; another scrunched up his eyes as though he were already facing the firing squad. "Only Edith Cavell kept her imperturbable calm." Of the 35 accused, Cavell and four others received the death penalty.

One of Diana Souhami's chief achievements is a painstaking reconstruction of the events leading to Cavell's execution which reveals the utter inadequacy of diplomatic and legal efforts made to save her. It is a tale of tardiness of response, unwillingness to disturb important men in their leisure hours, and supine bureaucrats. Souhami is also strong on the fall-out of Cavell's execution, which backfired on the German authorities. The 49-year-old nurse no one could be bothered to get out of bed to save provided, by her death, a massive propaganda advantage for the Allies. The slogan (and accompanying images) "Remember Edith Cavell", doubled recruitment in Britain from 5,000 to 10,000 for eight consecutive weeks. Souhami asserts that the "Cavell affair", as much as the sinking of the Lusitania, contributed to America entering the war.

Cavell's own opposition to war was conveniently ignored, and her words on the eve of her death - "I realise that patriotism is not enough. I must have no hatred or bitterness towards anyone" - were overlooked in favour of her more useful image as a martyr "for King and Country". Her own conduct and composure in the face of death spoke of a certainty that she was doing the right thing. When Stirling Gahan, the priest sent to pray with her prior to her execution, arrived at the prison, the guard, unprompted, told him she was a fine woman. "Like this," he said, and stiffened his back.

There are very few examples of Cavell's stoical humour, but her parting words to Gahan serve. He had been with her for an hour and her execution was fixed for dawn. He thought he had better go. "You will want to rest," he said. "Yes," she countered, "I have to be up at five."