Medieval literature contains a pair of antithetical tales of erotic betrayal that have proved fascinating ever since. The first deals with Tristan and Isolde, the iconic adulterers, whose desire for each other attains an other-worldliness bordering on the religious: it appealed to the Romantics, Wagner above all. The second deals with Troilus and his love for a woman whose name shifts with each retelling. Shakespeare, the last major writer to tackle their story, calls her Cressida. His predecessors include Boccaccio, Chaucer and Robert Henryson and they knew her variously as Briseida, Criseida, Criseyde and Cresseid. The tale's psychological realism and tacit equation of the casualties of love with those of war struck deep chords, meanwhile, in the second half of the 20th century and still do at the start of the 21st. Shakespeare's Troilus and Cressida, once dismissed as a failure, is now accepted as a great work. Chaucer's Troilus and Criseyde was recently adapted for radio, and this year's Edinburgh festival sees the first ever staging of Henryson's The Testament of Cresseid.
A late accretion to the Trojan legend, the Troilus story seems simple enough. Troilus, a son of King Priam, embarks on an affair with Cressida, the daughter of Calchas, a Trojan deserter to the Greek cause. Anxious for the return of Antenor, a Trojan warrior captured by the Greeks, an exchange of prisoners is arranged, during which it is agreed that Cressida should be taken from Troy and handed over to her father. The distraught lovers swear constancy, but while Troilus remains true to his vows, Cressida, once in the Greek camp, takes the captain Diomede as a lover. A number of features regularly associated with the story are by no means common to all its versions. There isn't always a go-between to bring the lovers together. Troilus's death at the hands of Achilles is sometimes not mentioned. Nor do we necessarily discover what happens to Cressida and Diomede, an omission that Henryson was anxious to rectify.
Given that much medieval literature strives for cosmic completeness, the story, with its emotional loose ends left untied, strikes us as starkly modern. From the 12th to the 17th century, however, the relationship between Troilus and Cressida was considered both emblematic of a world riven by fortune, mutability and transience, and central to a historical narrative that raised questions about fate and free will. The story didn't only encapsulate the vagaries of desire, it tackled the nature and meaning of suffering, and the absence of grace for those unable to hear Christ's message.
Medieval writers understood the Trojan war to be historical fact, though they also distrusted Homer as a source, preferring instead two shadowy books in Latin from around the 4th century AD. Attributed to the Trojan Dares Phrygius and the Greek Dictys Cretensis, both were believed to be translations of lost Greek or Phoenecian originals, and though spurious, were accepted as eye-witness accounts of the war. Dares gives us the first recognisable descriptions of Troilus, Briseida and Diomede. There is as yet no relationship between them, but it is here that we are told about Troilus's youthful beauty, Briseida's slightly imperfect glamour and Diomede's heavy-bodied masculinity. These traits stick with the protagonists up to Chaucer and beyond.
We first encounter their triadic relationship in Benoît de Sainte-Maure's Le Roman de Troie, written around 1160 for the French-speaking court of Henry II and his wife Eleanor of Aquitaine. Everyone who was anyone in medieval or Renaissance literature was familiar with this vast, seminal poem, whether in its original French or in one of the numerous translations or adaptations that it spawned. Benoît's achievement was not only to fashion a narrative from Dares's unconnected characters, but to make it a microcosm of the entire Trojan story. Its centrality is literal, since Benoît places much of it at the halfway mark of his poem.
Calchas has left Troy on hearing an oracle of its destruction, and his presence is a constant reminder that the city's fall is irrevocable. Troilus's name, meanwhile, means "little Troy", and Briseida's infidelity with Diomede not only echoes the conflict's origins in Helen's betrayal of Menelaus with Paris, but carries intimations of the greater treachery that will bring about the war's end. Her exchange for Antenor is a fulcrum around which the whole narrative swings. Benoît follows Dares and Dictys in claiming that Antenor betrayed Troy to the Greeks, an act which earned him a reputation for infamy surpassed only, according to medieval historians, by that of Judas Iscariot. Benoît cannily presents Diomede as sexier than Troilus, so that Briseida's capitulation, like everything else in this poem, carries with it a sense of inevitability.
Boccaccio's Il Filostrato, written around 1335, rejects Benoît's determinism, replacing the apparatus of epic with intimate domesticity as part of a dazzling, sophisticated display of erotic manipulation, played out, or so we are asked to believe, for an audience of one. The title means "Love-struck", and refers not only to Troilo, but to the anonymous, probably autobiographical narrator, who has been dumped by his mistress, and is writing her an extended poem comparing his misery to Troilo's in the hope that he can persuade her to return. The war alternately throbs away in the background and jolts the characters from their erotic idylls and back to reality. Where Benoît gives us no details of the start of the lovers' relationship, Boccaccio presents us with an extended account of their growing desire for one another and a description of their rapturous sexual encounters.
In the process, he invented the often disquieting figure of Pandaro, Criseida's cousin and the go-between who brings the lovers together, but whose failures of emotional imagination also exacerbate Troilo's anguish after Criseida's desertion. Chaucer and Shakespeare subsequently made him her uncle, and turned him into an ageing roué. In Il Filostrato, however, he's an attractive young man, roughly Troilo's age, professing love for an unnamed - or possibly non-existent - woman, and disturbingly given to vicarious participation in other men's emotional and sexual lives. Criseida, meanwhile, is a widow, and therefore, in Boccaccio's eyes, an experienced, practical woman, who muses on her right to erotic pleasure without remarrying. Once she has encountered Diomede, whom she considers as being "by nature inclined to love", Boccaccio leaves her to focus entirely on Troilo's growing awareness of her betrayal and his increasing emotional vulnerability.
Chaucer's discovery of Il Filostrato was the trigger for his own Troilus and Criseyde, completed in the mid 1380s. The two works are frequently compared, usually to emphasise the expansive subtlety of Chaucer's version. As in Boccaccio, Criseyde is a widow capable of voicing her emotional and sexual longings. Yet in Chaucer we are more conscious of how her desires are circumscribed by a world that not only wages war for women but uses them for political trade. Her relationship with Diomede, which she views as emblematic of her own moral failure, is also motivated by a need for survival. Chaucer's Troilus has lost the tendency to self-pity that we find in Boccaccio's Troilo, and his expressions of grief have an often excruciating intensity.
Much of the poem's force, however, derives from its authorial stance. Chaucer's ability to swerve from the demotic to the spiritual allows him both to view his characters through the eyes of a knowing, slightly world-weary narrator and to surround their story with metaphysics of considerable complexity. Where Boccaccio is consistently, and gloriously worldly, Chaucer presents his pagan characters as possessing insights into a Christian revelation that is forever beyond their grasp. Troilus talks to Pandarus as if he is in the confessional, and after Criseyde's desertion adopts a standpoint derived from the 6th century philosopher Boethius. The characters are subject to fortune as God's instrument of earthly transience, yet are unable to understand the counterbalancing force of his grace. The ending is extraordinary. After Troilus's death at the hands of Achilles, his soul ascends to the eighth sphere of heaven, where he is able to look down on the Earth and laugh at all its woes before being guided by Mercury to his last resting place. The final palinode contrasts the fragility of earthly desire with the eternal love of God.
Troilus and Criseyde's greatness is often emphasised at the expense of both Boccaccio's originality and the furious power of Henryson's The Testament of Cresseid, written in Scots around 1492 and all too frequently referred to, erroneously, as a sequel to Chaucer's poem. In chronological terms, its narrative runs parallel to Chaucer's fifth and final book. Henryson invents a horrific close to Cresseid's life, and in the process takes the story's moral and metaphysical questioning to startling levels. Dumped by Diomede, Cresseid turns to the "court commoun" of prostitution, before reviling Venus and Cupid for ruining her life. In one of the poem's many disquieting set pieces, the classical gods associated with the seven planets of medieval astronomy and astrology descend from their spheres and infect her with leprosy as punishment for blasphemy. The lovers are permitted one horrendous final meeting. Cresseid is reduced to earning her living by begging indiscriminately from Greek and Trojan soldiers as they return from battle. Troilus one day drops his purse into her lap. Neither recognises the other, though the leper-woman vaguely reminds Troilus of his lost love. Cresseid dies after being told his identity, but not before she has returned a ring that Troilus once gave her. At the poem's close we find Troilus, his love untouched by circumstances, devotedly, perhaps obsessively, tending Cresseid's grave.
The remorseless narrative is balanced by deep ambiguities within the poem. Leprosy was believed to be sexually transmitted in the late middle ages, though the anguish and disfigurement it entailed was also thought to bring the sufferer into a close relationship with God. Anger and compassion pulse through Henryson's verse. Cresseid's extreme suffering is an outrage greater than any offence she may have committed. The morally balanced medieval world-view is beginning to buckle and break. When we encounter Troilus and Cressida in Shakespeare, we find them teetering on the brink of nihilism. Even here, they are shrouded in ambiguities: once read as an examination of an idealist's catastrophic passion for a tart, the play is nowadays viewed as a study of the futility of war, in which female sexuality has become a primary instrument of survival.
By the start of the 17th century, Troilus and Cressida had become synonymous with fidelity and infidelity respectively. Shakespeare makes frequent allusion to them away from the play that carries their names, most famously, perhaps, in The Merchant of Venice, where their names, introduced as exemplars of yearning into the love scene between Lorenzo and Jessica, make the air vibrate with sudden unease. This famous passage was hugely admired in the 19th century: in his opera Les Troyens, Berlioz quotes it at the height of the love duet between Dido and Aeneas, where our foreknowledge of Aeneas's eventual betrayal of Dido gives it a pungent irony. Post-Shakespearian versions of the Troilus story have been few and far between. William Walton's 1954 opera Troilus and Cressida - which cited Chaucer as its source despite the Shakespearian title - is a notable exception: an uneven score, it presents Cressida very much as the victim of a violent male world that both tricks and psychologically batters her into betraying Troilus. That the opera is often described as simplistic is testament to the complexity of its medieval and Shakespearian versions of the Troilus story and the fascination they continue to exert.
• The Testament of Cresseid is at the Hub, Edinburgh, from August 29. Box office: 0131 473 2000.
