There is little in the world that Josh Gibson, a young American research assistant, loves more than flea markets, junk shops and house clearances, particularly those selling old papers, scrapbooks and magazines. So when, in the spring of 2007, living in Paris with his wife Sarah, he heard that a flea market was to be held on the Boulevard Arago, just next door to his apartment, he decided to go in search of hidden treasure. Riffling through piles of papers, stacked at random at the back of one stall, he pulled out a large plastic sleeve. It appeared to be full of tantalising scraps—a shopping bag from a long closed store, ancient book jackets, bits and pieces of memorabilia from an earlier time. Rather than haggling over individual items, he settled on €10 and took the lot.

Later that night, he took a quick look at his haul. It was only then that he noticed, buried under other items, a small bundle of hand-written letters, neat and perfectly legible, though the paper itself was frayed and yellowing. With them was a photograph album, full of snaps of a man and a woman and the sights of Paris, and a number of typed police reports and witness statements. By the time he finally went to bed, he realised that he had stumbled on a remarkable find, a love story between a German soldier and a French secretary, during the four years of the occupation of France and Vichy rule. Here, in these pages of faded paper, was a tale of passion, intrigue, collaboration and rough justice. Though how these letters had got there, and why, and who had sold them, no one knows.

One day an e-mail arrived in my inbox. Knowing of my interest in wartime France (I had recently published "A Train in Winter", about 230 women sent on the same train to Auschwitz in January 1943), he wrote to ask whether I would like to pursue the story. Both the German soldier and the French secretary were dead, but a nephew of the soldier had seen the blog that Josh had written about his discovery, and was willing to provide more information. A cousin of the secretary was also happy to contribute. I went to Paris and met them both—agreeing, to protect their families, that I should write about everyone under false names. The letters Josh discovered had provided the bones of the story; what they told me added some flesh.

For all the immense amount of research done since the 1970s on wartime France, curiously little has ever been written about relationships between occupiers and occupied, beyond what can be imagined from the bald statistics about shaven-headed women and illegitimate children. About real and lasting love affairs between the two, the archives are silent.

It was at the World Exposition in Paris in 1937 that Johann and Lisette later claimed to have met, though the references to this first encounter are frustratingly vague. Johann, who was 28, had come from Saarbrücken, the industrial town in the Saar where his grandfather had settled in the 1880s to work in a steel factory. Unlike his young brother, Rolf, who was studious and conscientious, Johann was gregarious, an adventurous, light-hearted young man with thick, glossy hair swept back, and an easy smile. He was talented, and thought of becoming an architect, but he was a man who seldom saw things through. Though married and the father of a small girl, Johann had a roving eye. Lisette lived with her parents near the Hôtel de Ville, where they were the concierges of one of Paris’s solid 19th-century apartment blocks, with inner courtyards and marble and parquet floors. She too was attractive, with wavy brown hair piled high; she was elegant, and loved hats. They made a handsome pair.

In the summer of 1940, Johann was among the second wave of German soldiers to reach Paris. The May invasion of France had brought fighting troops, tall, fit, healthy-looking men, goose-stepping victoriously down the Champs-Elysées in their magnificent leather boots and grey-green uniforms. The year of the phoney war, and the sense of foreboding and possible shortages, had made the city and its inhabitants somewhat shabby and muted. And though in their wake had come the shadowy and sinister forces of the Gestapo, soon to be ferreting out resisters, communists and Jews, General von Stülpnagel, military governor of Paris and a soldier of the old school, had made it clear to his men that they must behave in a civilised manner, not least because the Germans planned to rule France with a remarkably small army of occupation, and so needed order, calm and the co-operation of the French. The civility was much appreciated. When they had got over the shock of their rapid and humiliating military defeat, the Parisians noted that their occupiers were not, as they had feared, brutal, rude or monstrous; on the contrary, they gave up their seats to elderly women, opened doors and handed out sweets. They were, the occupied told each other, perfectly "correct". As François Mauriac, just beginning work on his barely veiled criticism of collaboration, "La Pharisienne", observed, the French women found them "as exciting as the Tour de France". When it was hot, the young men took off their shirts and preened in the sun.

Johann, who was now 31, arrived in Paris in August as part of the auxiliary forces of the Wehrmacht, though precisely what he did is not known. His French was excellent, and he is thought to have been using his skills as an interpreter. He was often out of uniform, and it was on one of these occasions that he seems to have bumped into Lisette, in a café somewhere in the city centre. There had apparently been no communication between them since their encounter at the World Exposition and Johann’s initial reaction seems to have been one of suspicion. From a letter written later, it would seem that he hesitated, wondering for a moment whether she might have become a prostitute, but when he asked the waitress whether she was often seen in the café and the waitress replied that she had never seen her before, he pulled up a chair and sat down. For her part, Lisette would later say that had he been in uniform she would have shrunk from renewing their friendship, as relations with the occupiers were much frowned on. But in the summer of 1940, 2m French men had been taken prisoner, and many French women were lonely. She was 27.

Soon, they became lovers. "Ma chérie!" Johann writes to her in his occasionally awkward French. "When you are in his arms he is very happy, always and ever your Johann!" There is no mention of his wife, nor of the fact that she has just given birth to a second child, a boy; and when this was revealed is not clear. Lisette now lived in a flat of her own, in another tall, imposing stone building, with little metal balconies and friezes of flowers and wreaths. Whenever he could get away from his duties, Johann visited her. On fine days, they did the sights together with all the other Germans who had arrived in Paris, and who had grown up to think of France as beautiful and culturally impressive, but at the same time insufficiently martial and somewhat decadent. "Gott in Frankreich?" (1929), Friedrich Sieburg’s bestselling book, warned France to adopt German ways in terms that would be familiar to Frenchmen today, lest it fail to find a place in the new European order. Paris had been put on to German time, swastikas flew from public buildings, signposts had been translated into German, and the city was bereft of birdsong since the destruction of the petrol dumps on the Seine estuary had wiped out most of the bird population. But for the rest, Paris was remarkably unchanged, the theatres, cafés and restaurants had all reopened and were thronged with Germans and French alike. To show how agreeable they were, German military bands gave free concerts of Beethoven and Wagner in the Tuileries and by Notre Dame.

When not climbing the Eiffel Tower—on foot, since the Resistance had put out the lifts—or strolling under the plane trees taking photographs of each other, or wandering through the very sorts of flea markets in which Josh would later find their letters, Johann and Lisette go dancing in the packed nightclubs. In the bi-weekly German newspaper, Der Deutsche Wegleiter, there was advice on places to visit, on race meetings and fashion boutiques. After Hitler’s whirlwind visit to Paris at the end of June, the Wehrmacht had coined a phrase, "Jeder Einmal nach Paris", to denote his wish that all German soldiers should have the chance to visit Paris for a holiday. With the French franc devalued and the German mark strong, Johann had money in his pocket; he and Lisette spent it on enjoying themselves. And on eating: as Simone de Beauvoir remarked of the Germans, never had so many people been seen "swallowing such quantities of food".

And, when Lisette and Johann cannot be together, they write to each other, loving letters full of longing. Sometimes he encloses a poem, with many references to roses and sunsets. One day, when Lisette is ill and has to cancel a meeting, he writes in despair: "My heart is very very sad…my little one, I am heartbroken!" He tells her that he wishes that she understood German, for then he would really be able to express the full depth of his desolation and unhappiness without her.

On Sundays, Johann and Lisette go to visit her parents, Jean and Françoise, who had taken to the charming Johann and now referred to him as their son-in-law, though whether they knew that he was already married is not known. What is known is that, like a significant number of French people, they admired Germany and hoped for a German victory. From the rest of her family there was outrage. Lisette was sleeping "avec un bôche!" In left-wing, working-class Paris, what would become known as "la collaboration horizontale" was a scandal. Cousins, uncles and aunts, decided to cut Lisette off.

She scarcely minded. Like Johann, she was in love. She knew perfectly well that Germans were not allowed to marry French girls—though sleeping with them was acceptable—that the Resistance regarded genuine love affairs between Germans and French as considerably worse than prostitution, and, like all Parisians, she knew Jean Texcier’s famous list of advice to the occupied, which included keeping them at arm’s length and not considering them to be anything but enemies. But she did not much care. In any case, like the other inhabitants of the capital, she was feeling the squeeze of occupation. Sugar, rice, soap, potatoes, meat, milk and cheese were now all rationed, and Parisians were growing hungry. Very conscious of her appearance, and always chic, she was finding it ever harder to find anything to wear. Leather, cotton and wool had all but vanished. The clacking of wooden soles on Parisian cobbles was becoming the defining sound of occupation. But Johann had access to luxuries, and her father Jean was beginning to dabble in the black market. A relationship between occupied and occupier, complicated, perilous, seductive, was starting to ensnare them all. What the French would call “le temps des autruches”, the time of ostriches, had begun.

In April 1941 , Johann, attached to the Generalkommando and probably part of the auxiliary force of linguists, economists, geographers and racial specialists sent in advance to prepare for the invasion of the Soviet Union, was posted to Poland. His parting from Lisette was painful. His first letter comes from a village "deep in Poland", where he was camping in an empty barn, sheltering from the rain and cold and dreaming of Paris. He carries Lisette’s photograph close to his heart, "burning like a warm fire". Protect me, he writes, "protect me when I am frightened and through the cold nights".

His second letter, written three weeks later, contains some dried wild flowers, "to tell you how much I love you and how unbearably I miss you". At night, he tells her, while his companions sleep, he cries, thinking about their time together. "You are so far away and the hope of seeing you soon is so small. It makes me mentally ill." Looking at her photograph, he keeps thinking about "the wonder of our love, the greatest love in the whole of Paris, in the whole of the world". Of his life in Poland, of the brutality of the German campaign, he says nothing, beyond expressing a belief that the Germans will soon win the war, after an attack "against England and America, against the Jews". He was still in his barn a few days later when the weather turned hot and sunny, and he sat outside, smelling the wild flowers and thinking of their future together. The east, he says, is calm.

But not for long. On June 22nd, Germany launched Operation Barbarossa against the Soviet Union, with over 3m men, the greatest land invasion of modern warfare. There was a pressing need for raw materials and an assumption that the Red Army would be quickly smashed. Early battles brought impressive German victories. Caught up in the advance, Johann tells Lisette that he is being sent to "another front" as a "military reporter", but just what his duties will be he does not say, preferring to fill his letter with descriptions of flowers. He has been promoted to lance corporal, and is advancing on Moscow. Soon, he says, "nous serons les vainqueurs!" With the letter, he includes a love poem, in German.

It was almost a month before Lisette heard from him again. Three of her letters—which are lost—have reached him, he tells her, and "my heart cries with happiness". The war, however, is becoming brutal, ten of his friends have been killed and many wounded. It is a "miracle" he is still alive. "I am too feeble to live," Johann writes, in one of his rare admissions of doubt, "too feeble to cope with the anguish of this war...But I must come back, and you must wait for me!!!...My little one, I love you so much!!" In August, he was still advancing, still alive. "Do not be sad," he writes. "I am not wounded and I am not ill." He was again sitting in the sun. "Do not ask the state of my heart. It has but one desire, to see you again..."

By October, the Germans had lost over 400,000 men, dead or wounded, and conditions in the advancing army were deteriorating. A month later the temperature had dropped to -8, and Johann, in his thin leather boots and summer uniform, was wearing a captured Russian hat. The final assault on Moscow, in which he seems to have taken part, began in mid-November, but it met well-equipped Soviet troops and the Germans were forced to retreat. There were many casualties.

And then, something impossibly lucky happens: Johann falls ill. So ill, with some kind of infection to his kidneys and possibly frostbite to his feet, that he is sent back to hospital in Ulm. His next letter to Lisette is jubilant. He is alive! He had been told that he will convalesce in Germany and that he will be stopping in Metz on the way, where she will be able to visit him; and that afterwards there is a good chance that he will be sent back to Paris. "Think, my little one," he writes. "Spring, you and me, a symphony of love."

I found far fewer letters from Lisette than from Johann in Josh’s bundle, and in all of them there is a more doubtful, anxious tone; she fears for their future. She was able to take leave from her job and in January 1942 they spent a few days together in Metz. The visit was followed by renewed outbursts of passion and love poems from Johann, and a poem from Lisette: "The train has left/Bearing away my love/My only love, I love you." They had evidently discussed the fact that liaisons between Germans and French in France were attracting increasing hostility for after she’s left he writes, "Don’t say that we are wicked, bad, no, no, you know that we could not do otherwise."

In March, a doctor certified that Johann was "no longer fit for service at the front". Preparing to return to Paris, he writes, "the end of our terrible separation, the realisation of the dream of our hearts". They resumed their affair. When in Paris, Johann visited Lisette twice a week, spending the night. When away on a mission he writes often and lovingly, telling her how beautiful he finds France and how he minded seeing it without her. From the south came a letter describing Nice in the Baie des Anges as "a bird trembling in its nest scented by thousands of flowers". Lisette’s answers are shorter and sometimes reproachful. Does he miss her? Does he really love her? She, too, was a poet, and like Johann sends verses about flowers and the transports of love.

At some point, Johann went on leave to Saarbrücken, where he saw his children and gave his small son a model railway set. His mother had told him that he seemed changed, and though he longed to confide in her, knowing that she had sensed that he was no longer close to "l'autre"—the other—he did not want to worry her. Lisette’s doubts have clearly resurfaced. "Collaboration?" writes Johann to her one Thursday evening in the winter of 1942. "I think it is an illusion. You have to love deeply to understand. Love alone is stronger than patriotism, a love like this one. I love France in you, and you will cherish Germany through me."

One of the more surprising aspects of the correspondence between Johann and Lisette is how detached both seem from the world around them, how sealed off in a bubble of their own heightened emotion. Neither ever refers to how the war is going, to what victory for Germany or France might do to their lives, to their future when the fighting stops. They never discuss the acute food shortages, the endless queues for rationed goods, or the fact that the winters of 1941 and 1942 were so long, and so exceptionally cold, that people in Paris died of frostbite inside their homes. Nor do they mention what was, from the spring of 1942, when Johann was back in Paris, the single most defining act of repression and persecution perpetrated by Vichy and the Germans, working together; though in this, Lisette’s family was deeply implicated.

Early in October 1940 a census of Jews was carried out in Paris. Soon after came the first Statut des Juifs, defining who, exactly, counted as Jewish, and banning Jews from a wide number of professions. In March 1941, Vichy, going ahead of German demands and declaring that what was needed was a "full-scale purification of Jews", set up a Commissariat Général des Questions Juives, whose job it was to confiscate—steal—Jewish property and eliminate Jews from the economic, social and cultural life of France. In May 1942 came the order for all Jews over the age of six in the occupied zone to wear a yellow star. By now, the deportations to the extermination camps in the east had begun and the roundups of Jews throughout France were gathering pace. To make Vichy’s task easier, rewards were offered to anyone who denounced a hidden Jew. This, the French were told, was a civic duty, since Jews, along with communists and freemasons, were public enemies. What amazed the Germans was the alacrity with which the French responded: it would later be calculated that three-and-a-half-million denunciations were handed in by the end of the war, many signed "un bon Français", or "a little woman who seeks only to do her duty".

One of Jean’s and Françoise’s tenants in their building was a Jewish doctor, a refugee from Nazi Germany, who had rented a flat there before the war, where he also had his surgery. In 1939, Dr Nathan Salezberger was called up to serve with the French army and taken prisoner on the Maginot line. Released at the armistice and sent to work in a military hospital in Toulouse, he wrote to ask Françoise to pack up a suitcase of his clothes—a blue suit, a long gabardine coat, three shirts, three pairs of socks, two pairs of shoes, six ties, 12 collars and a hat—and give it to his brother Melchior, who lived nearby, for him to send to Toulouse. Melchior duly collected the packed suitcase. But when Dr Salezberger opened it, he found that it was full of turnips and carrots. His clothes, apparently, were now either on Jean’s back, or had been sold on the black market.

Much worse followed. Soon after the hunt for hidden Jews began in Paris, Melchior and his wife Hélène left their own apartment and moved into an attic at the top of Jean’s and Françoise’s building, where they hoped no one would think to look for them. They were hiding there one day when the Gestapo arrived, ushered up the stairs by Françoise, who showed them which door to break down. Melchior and Hélène were taken to Drancy, the vast, filthy, overcrowded and terrifying holding camp for Jews on the edge of Paris. According to records found after the war, they were on either Convoi 52, which left for Sobibor, the extermination camp on the Ukrainian border, on March 23rd 1943, or on Convoi 53, which left two days later. In 1945 there was no survivor from Convoi 52’s 780 deportees; from Convoi 53, which had taken 527 men, 472 women and 49 children, five people came home. Melchior and Hélène were not among them.

On July 10th 1944 , just over a month after the allied landings in Normandy, French railway workers called a strike in Paris. Four days later, on Bastille Day, 100,000 people took to the streets in a mass demonstration. Johann was ordered to join the newly formed Regiment Majestic and sent to guard the railway line surrounding the city. On August 25th the first columns of the Second French Armoured Division entered Paris from the south and the west. As the Germans began to pack and leave—in what the diarist Jean Galtier-Boissière called "la grande fuite des Fritz"—Johann, still with his unit, was sent in an armoured car to the fort at Aubervilliers with orders to defend it to the end. The French forces, the FFI, attacked, and his companions were killed, but in the confusion Johann managed to hide in a loft, where he found a suit of civilian clothes. He changed out of his uniform and escaped, prudently taking with him an engine driver’s cap.

Having stolen a bicycle, he reached the Porte de Pantin, where the FFI were busy erecting barricades to prevent the remaining Germans from escaping. Berated for not helping, his French so fluent that he was readily taken for an Alsatian, Johann joined them. Later, still on his bicycle, he made his way to Jean and Françoise who urged him to find somewhere else to hide. He slept the night in the Bois de Boulogne and next day set out in search of Lisette, who seemed to have disappeared. It was not until September 10th that he discovered that she had been arrested for "collaboration with the enemy". For a moment, he declared later, "I was completely thrown. But then, despite my extreme anxiety, the strong feelings I felt for this woman gave me the strength to wait for her release." He planned to turn himself in once he knew Lisette was all right.

In due course, Lisette was freed; but the state that she was in must have shocked Johann profoundly. Some 20,000 French women, accused of relations with the enemy, had their hair shaved in the first frenzied weeks of liberation, when scapegoats were needed to expunge, in purifying rituals, the shameful years of occupation. Lisette was one of them. Sleeping with Germans, it was decided, was an act of collaboration. But she was spared the horror of being branded with a swastika or paraded through the streets half-naked. When her employers refused to take her back, Johann gave her what money he had, and they shut themselves up in her flat, happy and relieved to be together, unsure what to do next.

The decision was taken out of their hands: Lisette received a summons to appear before the police. Courts of Justice had been set up to hear cases of collaboration. Early on the morning of December 20th, after a last sleepless night with Johann, Lisette went off to the police headquarters near the Parc Monceau. Johann sat on a bench and waited. When, by 3.30, she had still not reappeared he went to find her friend Annette, who had earlier sheltered them. Annette agreed to go and see what was happening. She reported that Lisette was with “deux messieurs”, having confessed everything. Johann understood, he said later, that "it was all over". He could have escaped; instead, he waited until two inspectors came to arrest him. His police file records that his eyes were blue, his hair brown, and that he had been employed as a draftsman by the Military High Command. Lisette, in her statement, admitted to having been his mistress since the summer of 1940.

Johann was handed over to the Americans, who transferred him to a prisoner-of-war camp at Laon in Picardy. For a long time he had no news of Lisette, which terrified him, since the last thing that he had heard, when they were together in police custody, was an inspector saying to her, "As for you, you will die." He was right to worry. Ten thousand people were arrested for collaboration and sent to Drancy, now the main holding place for those suspected of having colluded with the Germans in Paris. Over half were eventually freed for lack of proof, but the others received prison sentences and were stripped of all civil rights. Before the wave of bitterness and desire for vengeance passed, a small number were executed. (As the actress Arletty defiantly and famously said, when accused of consorting with German officers, "My heart belongs to France, but my arse is international.")

We know what happened to Jean and Françoise from the police reports, mysteriously included in Josh’s bundle of documents—short, typed, yellowing statements given to an Inspector Chaillet. When Dr Salezberger returned to Paris, he found that his flat had been completely emptied. He lodged a complaint against the concierge and his wife, for stealing his clothes and replacing them with carrots and turnips; for looting his belongings; and for denouncing his brother and sister-in-law. Françoise and Jean were arrested. Swearing at the police when they came to get her, and telling them that she had hoped that Germany would win the war, Françoise disappeared into custody. Jean insisted that he had known nothing about his daughter’s German lover and that, having quarrelled with his wife for insisting on listening to the banned BBC over the radio, he had moved out and gone to live in an empty flat on the sixth floor. He added, helpfully, that he had indeed heard his wife say that she thought the Germans were every bit as good as the French. He admitted to having stolen Dr Salezberger’s electrical equipment, his glass cabinet, his chair and stool and his carpets. But what happened to Jean and Françoise after this is not known.

As for Lisette, she did not stay long in Drancy, which was as filthy and overcrowded with supposed collaborators as it had been during the years of occupation. Able to produce documents proving that at one point she had helped the Resistance draw up some lists useful to them, she was released and allowed home. When she discovered that Johann was in the camp at Laon, and that German prisoners were not allowed to consort with French women, though their American guards were, she got herself invited to one of their weekly dances where she was able at least to see him. Johann, with his good French and passable English, and his genial manner, had been made barman of the American mess. When Lisette leaves, he writes of his happiness and relief, decorating the pages with drawings of prison bars and barbed wire: "We have found each other! This time for ever!" Learning that there was a convent orphanage full of hungry children nearby, he had arranged for the food left over by the Americans to be collected by the mother superior. Laon, by all accounts, was a decent place, unlike many of the camps. The Allies held 800,000 German prisoners in France, in conditions that in some places were so bad that the men were dying of exposure, disease and malnutrition.

In December 1946, Johann was sent back to Germany, to a camp in Baden-Baden, to await release. Lisette was in despair. Her heart, she writes to him, is "full of anguish". She tells him that she hates thinking of him as a prisoner, but that, without him and alone in Paris, she feels a prisoner herself. A few days later she writes again, a five-page, frantic letter. It is a Sunday and she is alone, cold and miserable. "My heart suffers and my body too," she writes, "and I cannot calm either one of them...Yes, my Johann, your kisses must revive me…Keep everything for me, you belong to me." She is growing unsure of his commitment. "When will I see you?" she writes. "I live in your shadow and will always be yours, yours alone, your little one." Whether Johann’s apparent lack of urgency in pressing her to join him indicates that he was by now tiring of his wartime passion, neither his nephew nor her cousin can confirm.

But early in 1947, without papers and wearing only thin shoes and clothes, she smuggled herself over the border and went to find him. She then returned to France, joined the French women’s auxiliary forces and got herself posted as a secretary to the Army of Occupation in Baden-Baden. She was very pretty and could be persuasive. "We are young," she writes to Johann. "Life owes us something." Having waited so long, she believed, she was entitled to some happiness.

The trouble, of course, is that there is no entitlement to happiness. On February 12th 1949, Johann and Lisette married. The divorce from Johann’s wife had been bitter, and neither of his children wished to see their father or to form any kind of relationship with their new stepmother. Lisette’s wider family, however, had forgiven her, and when they came to visit her in Germany they much enjoyed being taken around the country by the charming, affable Johann, who entertained them by making up poems as they drove about.

But Johann had not lost his eye for the girls. Nor was he as hard-working as Lisette had imagined him to be. Before she joined him, he had used his new skills as bartender, and his command of French, to work in a series of restaurants and hotels catering to the occupying forces. After they married, they became managers of a hotel and restaurant on Lake Constance. According to her cousin, Lisette was an excellent cook. But Johann was lazy, preferring to spend his time with the pretty young tourists who came to the lake in the summer months, rather than keeping his eye on the books. When an employee made off with a considerable sum of money, they were forced to leave the hotel. Bankruptcy now stalked them, as they moved ever down the scale, from café to bed and breakfast to campsite and souvenir shop. From time to time, Lisette returned to Paris, to window-shop for the elegant clothes she so loved, but she had cut herself off from her old friends, ashamed of the past, too proud to admit that her great dream had turned out to be a failure. She would not leave Johann, she told her cousin, because, for all his infidelities, he remained "my great love". The war, she said, had ruined the men who fought in it. It explained why Johann seemed to feel that he had a right to be unfaithful. They never had a child.

Johann died, aged 78, in 1986. Lisette lived on another 14 years, but refused to return to France, though her spoken German never lost its strong foreign accent and she remained French at heart. When she died, Johann’s nephew, to whom she had become close, buried her in a graveyard on the border between France and Germany, so that she could look out, in death, towards the country she had loved and betrayed. And there this story might have been buried too, had it not been for an American research assistant and his love of old papers.