Cleopatra - the last queen of Egypt; one of the most formidable enemies Rome ever faced; the woman whose two husbands, both of whom were also her brothers, died in their teens (one in battle against her, the other possibly murdered on her orders); the lover who thereafter chose her own partners with an eye not only to pleasure, but also to the augmentation of her own power. She appears on the Glyndebourne stage this summer, portrayed by Danielle de Niese, in an unfamiliar character: that of a sweet helpless girl desperately in need of a male protector. Handel's opera Giulio Cesare (libretto by Nicola Haym) introduces a surprising vision of Cleopatra. She is recognisably linked to the Cleopatra of Dryden's All for Love, a fluttery creature who describes herself as a "silly, harmless household dove". But she bears almost no resemblance to the more familiar Shakespearean "serpent of old Nile" currently to be seen at the Globe, where Frances Barber plays up her violence, forcing the unwelcome messenger's hand down on to a brazier full of hot coals, and at Stratford, where Harriet Walter endows her with fierce intelligence and sorrowful majesty.

All legends have a tendency to mutate, to be reshaped in each successive era according to the prejudices and preoccupations of those who retell the tale. But Cleopatra's is more than usually protean. It was first formulated in her own lifetime by her enemies' propaganda. Its primary purpose was to discredit her lover Mark Antony.

Cleopatra and Antony had formed a partnership that was as much a political alliance between two mutually useful potentates as it was a love affair. But the story, as Roman poets and historians tell it, was that Antony had become so besotted with the queen of Egypt that he was willing to give up his chance of ruling Rome in order to enjoy the pleasures of her bed. So Antony, the canny politician and commander with empire-building ambitions to rival Alexander's, was reinvented as a degenerate hedonist and a traitor to Rome. As a by-product of that successful exercise in news manipulation, Cleopatra was cast as the woman for whose love's sake the world would be well lost.

Cleopatra - the gratification of every conceivable desire - has been repeatedly reimagined by writers, artists and film-makers in accordance with desires of their own. She was one of the most powerful women in the ancient world, and she was defined by the Romans and their heirs as the foreigner - at once the menacing stranger and the temptress, offering the chance of escape from the tedious limitations of one's own known world. So sexual and racial politics have shaped the variations on her story, transforming her from serpent to dove and back again to suit her public's yearnings and fears.

Her moral status fluctuates. Cecil B de Mille offered the leading role in his sumptuous movie about her to Claudette Colbert with the words: "How would you like to play the wickedest woman in history?" His question anticipated the answer: "Very much indeed, please." "Wicked" was already, in the 1930s, a term of approbation meaning sexy, edgy, thrilling, an infinitely more alluring epithet than boring old "good": the hypocrisy at the heart of our culture has been part of Cleopatra's legend for most of 2,000 years. But back in 1380, Geoffrey Chaucer made Cleopatra the first heroine of his Legend of Good Women. To him and his contemporaries she was the paragon of feminine virtue, the proof of her goodness being that she didn't wish to outlive her man. In an age when love-matches were rare and widowhood the only condition in which a woman could be truly independent, men, it appears, found it hard to trust their wives.

The emphasis of her story wavers as often as her claim to virtue. To the Renaissance painters, it was about sex and money. They produced quasi-pornographic images of her suicide. Although all the ancient historians agree that Cleopatra had herself dressed in all her royal robes before applying the asp to her arm, artists almost invariably picture her dying in the nude, with the snake at her breast. Or they show her presiding over a magnificent banquet, in the act of demonstrating her prodigality and her wealth by drinking down a pearl dissolved in vinegar.

To the English and German dramatists who took up the theme after Shakespeare, it was about the relative status of wives and mistresses. In the Protestant cultures of post-Reformation Europe there was a lively debate about the new ideal of companionate marriage, one which spilled over into Cleopatra's story: the reason Dryden's Cleopatra feels so feeble is that she lacks a wedding ring. Next, in versions written in the period leading up to the American and French revolutions, the story became about the clash between rival systems of government. In some dramas (notably those produced under Louis XIV and his successors), Cleopatra and Antony represent the feudal nobility as opposed to Octavius's centralising and modernising regime. In others, Cleopatra stands for the decadence (and romance) of ancient monarchy contrasted with Roman republicanism.

The storyline shifts. So does Cleopatra's appearance. For several hundred years she was blonde. She was a famous beauty, and so medieval poets ascribed all the conventional attributes of beauty to her: hair like spun gold, sky-blue eyes and breasts "as white as ivory billiard balls". The tradition was persistent. Shakespeare's Cleopatra may have been darkened by "Phoebus's amorous pinches", but in Tiepolo's magnificent frescoes in the Palazzo Labia in Venice she is as pearly-pale as the earring she is about to drop into her gilded cup, with albino eyelashes and opalescent breasts. It wasn't until the very end of the 18th century, the period when Napoleon sent his troops and his scholars to Egypt, that Cleopatra's exoticism became once more (as it had been in her lifetime) the most important thing about her. Delacroix painted her as a kind of Gypsy fortune-teller, dark-eyed and tousle-haired.

Over the next century, as the European powers scrambled for territory in Africa and the Middle East, Cleopatra's legend became the vehicle for theories about racial difference and justifications of imperialism. Artists represented her as an enticing Turkish dancing girl in gauzy harem pants and sparkly bra; lolling indolently on a divan, she became representative of a terminally decadent culture ripe for annexation by a benignly energetic western power. In the latter pose, she is usually surrounded by slaves bearing cups of sherbet and peacock-feather fans. Or, in many cases, writhing in agony on the floor.

In the 19th century the Cleopatra plot ceased to be the familiar, more or less historical one of Antony, Actium and the asp. In 1837 Pushkin revived a scurrilous piece of fourth-century gossip alleging that Cleopatra used to offer a night in her bed to any man willing to pay for the privilege with his life. He expanded on the theme. Cleopatra, presiding over a banquet in a mood of idle boredom, makes her terrible offer. Her courtiers are aghast, but a line of men beg for the prize. She rejects princes and generals, accepting instead a fresh virgin boy, whose execution she watches with relish the following dawn.

Romantics and decadents alike adored the story. Cleopatra was reborn as the femme fatale, the personification of the bourgeois male's sexual guilt and the realisation of his most deliciously painful self-castigating fantasies. Plutarch had recorded that, in preparation for her own death, she tested poisons on her household slaves: the scene was represented repeatedly on canvas and on the stage, while Pushkin's scenario became the basis for dozens of later versions. The tragedy of Cleopatra in which Sarah Bernhardt starred repeatedly over three decades (doggedly continuing even after she had lost a leg) was not Shakespeare's, but one written to the actress's order by Victorien Sardou, in which the queen is a sadistic voluptuary given to performing elaborate striptease. Meanwhile, the bestselling novelist H Rider Haggard came up with a new twist when he revealed that the youth Cleopatra took to bed was actually her own son.

Cleopatra had become the personification of vice, flouter of every convention, breacher of every taboo. She was barely human. Algernon Swinburne, in an essay that is part art criticism, part masochist reverie, enthused about Michelangelo's drawing in which queen and asp seem to fuse into one Medusa-like being, while Gustave Flaubert called her "the pale creature with a fiery eye, the viper of the Nile who smothers with an embrace". Ruthless, beautiful, bestial, this fantastic Cleopatra seemed to offer Europeans, feeling cramped in an increasingly regulated society, an escape into a Nietzschean wonderland of moral irresponsibility and violent pleasure.

By the time the Ballets Russes staged Fokine's Cleopatra in Paris in 1909, she had become the figure of death. The dancer who played her, Ida Rubinstein, was carried on in a sarcophagus, wrapped mummy-fashion in yards of multicoloured gauze from which she was gradually unwound, her face chalk-white, her hair bright blue.

The fantasy of the death-dealing vamp flourishes in peacetime. In the face of the 20th century's wars it came to seem a bit of a joke, and a sick joke at that. In 1917 the movie Cleopatra, starring Theda Bara (described by the Fox publicity team as the "Ishmaelite of femininity" and the "torpedo of domesticity"), bombed at the box office. In a world where young men were being slaughtered en masse, the femme fatale was redundant. Bernard Shaw scoffed at the idea of sublime passion, and wrote a Caesar and Cleopatra in which the queen of Egypt is a petulant teenager. Claudette Colbert played her as a flirty good-time girl and, in 1945, Vivien Leigh brought to the part, in Kenneth Tynan's words, "the daintiness of a debutante called upon to dismember a stag". Cleopatra had become camp.

In the notorious 1963 movie she arrives in Rome on a mobile sphinx as high as the Senate house, accompanied by belly dancers, whirling dervishes, wheeled pyramids that open up to release flocks of white doves, scores of chariots, archers and armies of well-oiled, buff attendants in fetching pink loincloths. Arriving before Caesar, Elizabeth Taylor, heavily made up in early 1960s style with lots of eyeliner, false lashes and pale lipstick, bows deeply, her bosom looming large around the edges of her deeply cut gold-lamé bodice. And then, looking up at Antony, she winks. The scandal generated by Taylor's on/off-screen affair with her Antony, Richard Burton, was gleefully welcomed by the film's producers. To the 19th-century Romantic, the wages of sin might be death. To the 20th-century entrepreneur, it was good publicity. To a postwar generation avid for life and pleasure, Cleopatra offered not a fatal passion, but history's best ever holiday romance.

In the last three decades of the 20th century, Cleopatra got serious again. She was allotted a role in a debate about race relations. Afrocentrist historians, led by Martin Bernal, argued that the culture of ancient Egypt had been played down by racist scholars unwilling to acknowledge that Greek civilisation, and therefore all subsequent western civilisation, had African origins: it became fashionable to describe the pharaohs as black.

But whatever colour the pharaohs were, Cleopatra was not one of them. She was the direct descendant on her father's side of one of Alexander's generals, a Macedonian Greek. We do not know who her mother was: her ethnic inheritance can't be fully established. But we do know that when the director of a 1990s production of Shakespeare's play, who had cast a black actress in the role of Cleopatra, talked about emphasising her "earthiness" and "the kind of non-European regality which allows someone to sit on the floor", she was imposing yet another set of anachronistic preconceptions on to the image of the Hellenistic queen. Already, only a few years later, that director's remarks sound insulting: we do not now think of "earthiness" and a reluctance to use the furniture as particularly "black" traits. In fact, Cleopatra shocked the republican Romans by sitting not on the floor, but on a throne of solid gold.

More recently, Cleopatra's Middle Eastern identity has come to seem even more interesting than her African one. In 1929 the Egyptian dramatist Ahmad Shawqui, a campaigner against the British authorities, made her a nationalist heroine struggling to defend her country's independence. The theme is ready for further development. No one retelling her story today could do so without an awareness that she was the ruler of what is now an Islamic state, at the moment of its invasion by a western superpower.

In Cleopatra's lifetime, racist Roman propaganda characterised Egyptians as self-indulgent, sex-fixated and unmanly in their readiness to treat women as their equals, while Romans congratulated themselves on their abstemiousness, the austerity of their religion and their readiness for war. The stereotypes are still recognisable, but their ascription has been reversed. A militant Islamicist from the region that Cleopatra and Antony once ruled must now think of the west much as Rome once thought of Cleopatra's Egypt.

Cleopatra is still changing, and she will continue to do so as long as her name is remembered. The forces that have repeatedly transformed her image, the forces of anger and anxiety and covert desire, are still at their lethal work in the world.