Napoleon and Josephine's volcanic romance was so overtly sexual - including her famous 'zigzag' technique - that it transfixed Europe. Now Ridley Scott's new film starring Joaquin Phoenix reveals the French emperor's devastation when they divorced

Theirs was the love affair of the century - the 18th century.

She, the spoilt daughter of a slave-owner, raised on a Caribbean plantation, who ate so much sugar that her teeth turned a grizzled black. He, the hollow-cheeked military upstart with an astonishing capacity for rudeness trumped only by his unwavering arrogance.

Their relationship was so tempestuous, dramatic and overtly sexual that it transfixed and alarmed French high society in equal measure, while the couple's exploits - in the boudoir and on the battlefield - have become the stuff of legend.

Her full name was Marie-Josephe-Rose de Tascher de La Pagerie. But she is remembered simply as Josephine, wife of Napoleon Bonaparte and Empress of France.

Yesterday saw the release of Ridley Scott's epic blockbuster biopic tracing the life of Napoleon (Joaquin Phoenix) and his marriage to Josephine (Vanessa Kirby, best-known for her brilliant portrayal of Princess Margaret in The Crown).

It is set to grip audiences, plastering the couple on billboards up and down the country - a haunting reminder of what might have been, had the Duke of Wellington lost to the vainglorious Corsican at Waterloo in 1815.

Director Sir Ridley, whose credits include Gladiator and Alien, points out that 10,400 books have been published about Napoleon's life: 'one every week since he died'. But far less has been written about the only woman he ever truly loved, and whose name would fill Napoleon's final breath.

Josephine - known to her family as Yeyette - was born on the tiny Caribbean island of Martinique in 1763. Her once-prosperous family was in decline, living in a crumbling mansion in the middle of their plantation, from where Josephine's father, drunkard gambler Gaspard, could see his 300 enslaved men, women and children toiling in the sugar-cane fields, watched over by brutal overseers.

In fact, Yeyette spent her infancy more in the company of slaves than friends or family, and she would regularly witness the terrible sight of Africans hauled off ships and taken to market, then branded, shackled and forced into work.

As a 16-year-old, Yeyette married wealthy Vicomte Alexandre de Beauharnais, three years her senior, though there was no pretence of affection or attraction from either side.

Perhaps unsurprisingly, after moving to France and having two children, the couple separated when Yeyette was just 22.

Yet it would be another decade before her historic match with Napoleon could begin.

In 1794, Yeyette, then in her early 30s, was in prison and facing execution. This was what would later be called the 'Reign of Terror' - a period in the French Revolution in which aristocrats and landowners faced violent persecution.

Her estranged husband, Alexandre, had been guillotined. She survived in her cell on scraps while her little pug, Fortuné, was trained to carry messages to and from her children on the outside.

Shortly before she would have faced her own appointment at the guillotine, Yeyette was saved when Robespierre, the architect of the Revolution, was deposed. 

She had been badly marked by her imprisonment: her youthful beauty had been despoiled, her teeth were further damaged and she was unable to bear any more children.

However, having suffered during the Terror brought her a certain social cachet.

As one observer opined, it was 'the height of good manners to be ruined — suspected, persecuted and above all imprisoned'.

The end of the Terror saw a flourish of wild abandon among the Parisian elite. Women far outnumbered men and Yeyette went from outcast to socialite almost overnight. She was quickly taken up by an older man, Paul Barras, commander of the Army of the Interior. 

Though they were lovers, this was an intensely pragmatic relationship. Both knew the political winds could change again quickly, and Barras was grooming a protege for power, a young man who could be his protector.

His name was Napoleon Bonaparte. Napoleon was making a spectacular rise through the ranks. Two years earlier, he had commanded an artillery detachment that drove the British Army out of Toulon. Barras introduced this ambitious soldier to his lover at a party, urging her to charm the younger man.

Naive and inexperienced with the opposite sex, Napoleon didn't imagine for a moment that this impossibly glamorous woman, six years his senior, was playing a game. She declared that she admired him, and he believed it without question. 

Never had he met a woman like her: most of his previous encounters had been clumsy fumblings with ladies of the night. He lost his virginity in a Paris brothel, apparently on the fourth attempt.

It wasn't long after meeting Josephine that the 26-year-old Napoleon had fallen hopelessly for the widow with two children. So began the volcanic romance that would spur Napoleon through Europe, throw him into fits of unbridled rage and entwine him in a marriage of unquenchable lust - until political realities took over.

The first time they made love in 1795, he wrote to her, dating the note 'seven in the morning' and revealing: 'I have woken up full of you. The memory of yesterday's intoxicating evening has given my senses no rest. 

'Sweet and incomparable Josephine... I draw from your lips and heart a flame that burns me. In three hours I shall see you. Until then, mio dolce amore [my sweet love], thousands of kisses, but don't kiss me, for your kisses set my soul on fire.'

They made love, they fought and argued, then they made love again. Napoleon renamed her Josephine - a contraction of her full name Marie-Josephe-Rose - and wrote to her: 'What is your strange power, incomparable Josephine? I give you three kisses, one on your heart, one on your lips, one on your eyes.'

Despite her misgivings - she told Barras that she thought she could do better than Bonaparte - they were married in a low-key ceremony, two days before Napoleon departed to conquer Italy in March 1796.

He wrote constant, graphic love letters while away that are so achingly hot, they smoulder: 'I am going to bed with my heart full of your adorable image. I would be so happy if I could help undress you, small shoulder, small white breast, supple, very firm. You know that I always remember the little visits... The little black forest. Kisses everywhere!' 

The 'little black forest' is certainly a euphemism, as is Napoleon's habit of referring to her genitals by the bizarre name 'Baron de Kepen'.

There is also the infamous suggestion that Napoleon preferred his wife to be 'unwashed' before making love. While this has been widely reported, it is probably a slur invented by the British as propaganda. If it were true, it would be more likely that he wanted to make sure she was not having affairs behind his back.

He was also entranced by a sexual technique Josephine employed which he called 'the zigzags', though quite what that entailed is unclear.

Such is the fascination with Napoleon's love life that it was even claimed that the doctor who performed his autopsy removed his penis, and sold it to the priest who had conducted the last rites. 

This was later apparently sold on to an American collector, although the provenance has never been confirmed. The shrivelled relic can now be found at the home of a New Jersey woman, Evan Lattimer, whose urologist father, an eclectic collector, bought it in 1977.

Josephine sent terse replies to his letters, which drove him wild with frustration. She also took a lover, a 23-year-old lieutenant in the Hussars named Hippolyte Charles - extraordinarily good-looking, with a chiselled face, fine blue eyes and dark hair. 'I think there is no one in the world who ties his cravat with more aplomb,' she wrote.

After months of prevaricating, she agreed to join her husband on campaign in Italy in June 1796. She took Hippolyte with her as her bodyguard, a pretence that fooled nobody except Napoleon.

It was only two years later, when Napoleon was invading Egypt, that his generals managed to convince him that Josephine had been unfaithful. In a fury, he threatened to divorce her. She begged forgiveness, and he paid off her debts which amounted to millions of pounds at today's values, but the balance of power in their marriage was altered for ever.

Now it was Napoleon who took mistresses, while Josephine was forced into faithful obedience that may well have had its own erotic value to him. He had dozens of women while on campaign and in Paris - actresses, courtiers, ladies-in-waiting, dancers and fashionable women. 

He demanded that they should not wear perfume and often wait for him undressed, so that matters could be speedily conducted. He also tended to lose interest as soon as he had conquered them.

Josephine was barred from his rooms but he would often come and tell her afterwards about his thoughts on the 'performance' of the ladies with, as one of her friends said, 'the most indecent openness'. He would also describe their 'physical imperfections and anatomical peculiarities' to both Josephine and his male courtiers.

Ultimately, it was Josephine's inability to bear his children that ended their marriage. Desperate for an heir, and stung by the mockery of his own family (who never liked Josephine), he conceived a son with a young married Polish woman, Marie Walewska.

This spurred him on, and after arranging to have his marriage to Josephine annulled, he organised another betrothal to the 18-year-old Archduchess Marie-Louise of Austria. 'I am marrying a womb,' he said sadly.

Such scathing misogyny was typical. He once told a woman that she looked as though she had just given birth, and also was reported to have said that 'a woman laughing is a woman conquered'.

And yet, the thought of separation from Josephine made him physically ill. At a divorce ceremony held at court in December 1809, he trembled so uncontrollably that his ministers feared he might faint.

Josephine arrived in a plain, white gown, bowing to each courtier who offered condolences. 'I doubt,' said one, 'whether any woman could have acted with such a perfect grace and tact.'

Napoleon proclaimed the divorce: 'God only knows what this resolve has cost my heart. But there is no sacrifice beyond my courage, if it is in the best interests of France. I have only gratitude to express for the devotion and tenderness of my well-beloved wife.' Then he wept.

'The Emperor will always be my dearest love,' she replied. 'I know how much this act, demanded by politics, and wider interests, has crushed his heart.'

The night of the divorce ceremony, she went to him in his bedroom and tried desperately to seduce him. Instead, he held her and cried tears of pure sadness.

Rumours of that sad parting, and how Napoleon had refused his ex-wife's final advance, became part of their legend.

When, in the early 20th century, a popular music-hall song was composed -  'Not tonight, Josephine' - it quickly became part of the assumed legend of the couple, although the original song may not have been about them at all.

Josephine was named Duchess of Navarre, allowed to retain her beloved chateau outside Paris, and remained on good terms with her former husband.

Her replacement may have granted him an heir, but when Napoleon - then in exile for the first time, on the Mediterranean island of Elba - learned of Josephine's death, most likely from pneumonia in 1814, he locked himself in his room for two days and refused to see anyone.

He later told a friend mournfully: 'I truly loved my Josephine, but I did not respect her.'

Napoleon died, probably of stomach cancer, on May 5, 1821, while in exile on the island of St Helena, off the coast of Africa. His final words were, 'La France, l'armée, tête d'armée, Joséphine' ('France, the army, head of the army, Josephine'). A fitting dedication to a great femme fatale, and the only woman the most power-hungry man in Europe had ever truly loved.

• Professor Kate Williams is the author of Josephine: Desire, Ambition, Napoleon, published by Hutchinson.