Tristan und Isolde had deep roots in the personal life of its author, and indeed no one coming to the work from whatever direction can entirely avoid them.* Glyndebourne's 2007 production of Wagner's Tristan und Isolde will be available to watch on demand on the Guardian from 26 December to 6 January 2013. Click here for details

Violet Urmana and Gary Lehman flank Esa-Pekka Salonen, conducting the Philharmonia in Tristan und Isolde. Photograph by Benjamin Ealovega; video imagery © Bill Viola

Dangerous Fascinations

Tristan und Isolde had deep roots in the personal life of its author and indeed no one coming to the work from whatever direction can entirely avoid them. Wagner first conceived Tristan in 1854 when he was almost wholly dependent on the patronage of a rich Swiss businessman, Otto Wesendonck, who not only advanced large sums of money to pay off his debts, but also provided Wagner and his long-suffering first wife Minna with a house close to his imposing estate in Zürich that was promptly named the "Asyl" by its grateful and impecunious recipient, then still a political refugee from Germany with a price on his head. Wagner responded, practically from the start of their acquaintance, by falling head-over-heels in love with Otto's attractive wife, Mathilde Wesendonck, who seems to have reciprocated with genuine affection that stretched, but cautiously did not overstep, the limits of upper-middle-class decorum. For his part, Wagner was smitten, his adulterous love for his patron's wife inevitably coming into painful conflict with his genuine loyalty and gratitude to his patron - a conflict similar to that between Tristan's passion for Isolde and his loyalty to his protector King Marke.

The situation undoubtedly left an indelible mark on the conception of Tristan, though at first with only a few hints of the final work's existential complexity. In a famous letter to Liszt from December 1854, Wagner wrote:

Since I have never enjoyed in life the actual happiness of love, I want to erect another monument to this most beautiful of all dreams, in which, from beginning to end, this love is going to satisfy its hunger properly for once. I have worked out a Tristan und Isolde in my head - the simplest and at the same time most full-blooded musical conception. Then I'll cover myself over with the "black flag" flying at the end so I can - die.

Wagner's mention of the black flag – a detail of the Tristan legend among the first to catch his attention - suggests that he originally had a different idea of the opera's outcome. In the legend, Tristan lies sick and incurable in the care of his wife, Isolde of the White Hands, while they await the arrival by ship of another Isolde, the Irish princess and wife of King Marke, whom Tristan secretly loves and whose powers alone can heal his wound. If the ship is flying a white flag, Isolde is on board; if the flag is black, she is not. As the ship comes into view, it is flying a white flag, whereupon Isolde of the White Hands, jealous of her husband's dependence on King Marke's wife, tells him the flag is black. Mistakenly believing that he will never again see the Isolde he really desires, Tristan dies of grief. The coded message to Liszt, fully aware of his friend's tangled emotional state, could not have been clearer: the beautiful dream of love, beyond the pale of a loveless marriage with my wife Minna, to whom I, Richard Wagner, am publicly bound ethically and legally, can at least be conjured up in a work of art, even though I will never enjoy the sight of the white flag, concealed for ever from me by the public bond to my lawful spouse.

The subsequent development of Tristan and Isolde in Wagner's imagination, however, gradually transformed the shadowy dilemma of his unhappy marriage and its petty jealousies into a wider examination of the tragedy of the human self as he saw it. The "beautiful dream of love" developed into a conundrum far beyond the confines of Wagner's private world, which, in short, could be described as a provocative equation between ecstatic love and the nightmare of delusion (even about the nature of love itself) and its resolution in death. The trivial modern phrase "to die for" used to express the succumbing to culinary bliss suddenly became appallingly literal. In the summer of 1856 Wagner wrote to August Röckel about an idea of a work "in my head", describing it as "love as terrible torment" (die Liebe als furchtbare Qual). The starkly-worded phrase left no doubt that the conception of Tristan und Isolde had altered significantly. Gone were the black/white flag and the jealousies of the rival Isoldes and nowhere was there mention any more of "the beautiful dream of love". What remained was a full-blooded musical conception, now saturated with an erotic hunger that could no longer be satisfied in life any more than it could be naively portrayed as a monument to a fulfilling erotic relationship permanently denied to its author. Tristan had changed from a relatively straightforward story of imagined love tragically compromised by jealousy into a desolate near-metaphysical exploration of the anguish of being-in-the-world brought about by the fatal intoxication of exquisite passion.

The golden age of private life

Perhaps because the story of how Wagner conceived Tristan und Isolde is not as well known as it should be, the idea that it is "the ultimate glorification of love" (a common description of it) is little more than a superficial cliché. In one sense the work really is the love story to end all love stories: if one understands it well, no other pair of lovers, however star-crossed, can ever be quite the same again. Not even Shakespeare's Romeo and Juliet, at first sight perhaps the work Tristan most closely resembles, is as bleak. But far from being "the ultimate glorification of love", Tristan at its core dares to present the notion that the secret, all-else-excluding passion between two people is just as much an illusion as the public political sphere of human relations that threatens to invade it. The lovers are martyrs in the name of freedom, not only from the public world that has caught them in its vice, but also from the torment of love itself, the dark allure of "night" they both eventually reject as vehemently as they do the bright realm of "day".

Nina Stemme as Isolde in Glyndebourne's 2003 production. Photo: Tristram Kenton for the Guardian.

The death-devoted rebellion against the public and the private in Tristan is undoubtedly one reason for its powerful effect on the 19th century. During that supposed golden age of private life the celebration of a realm of dreams and conflict served as the bulwark of reality and law. Yet the arc that stretched from the social engineering of the Jacobins in France in the 1790s to the cult of inwardness and aestheticism in fin-de-siècle Europe, when the influence of Tristan was at its height, included in its range significant doubts about the equilibrium between the public and private spheres that liberal policy was meant to have achieved. In a sense it is precisely this supposed equilibrium that is under attack in Tristan. Already in the first act, it is the pressure of a public event - Tristan's delivery of "Ireland's daughter, loving and wild" (the sailor's apt description of Isolde at the start of the opera as Cornwall's future queen) that practically encourages the lovers' fateful recognition of their hidden passion. After that, the public and the private retain their tragic dependency on each other almost to the very end.

Tristan and Isolde has not become one of the most alluring planets in the operatic universe just because it is a love story in extremis. The overly simple image only disguises the deep trauma of desire in two individuals in the opera, whose fated relation to each other and the external events that condition it are really beyond their control. Tristan was undoubtedly for Wagner the working out of an existential predicament, which compelled him not only to write some of the most searing and emotionally disturbing music ever heard in the theatre, but also to compose it remarkably quickly in three years (1857-59) when - as he later told friends - every fibre, every nerve in his body, was tingling and alive.

Ever since, its tale of illicit sexual attraction and the orgasmic sensuality of its music have held the Western world in thrall. Bernard Shaw once observed that Wagner retraced "poetic love" to its "alleged origin in sexual passion, the emotional phenomena of which he has expressed in music with a frankness and forcible naturalness which would have possibly scandalized Shelley." By that Shaw meant that Wagner's translation of the emotions that accompany the union of a pair of lovers into music posed a moral as well as a musical challenge for 19th-century audiences that went far beyond the notion of tragedy inherited from the Greeks and its emphasis on a communal celebration of human concerns. Indeed one small historical detail illustrating the point that Tristan in the minds of some had already begun to invade the official codes of public and private life even before it was revealed to the world is the fact that the Duchess Sophie of Bavaria was not allowed to attend the first performance of Tristan in Munich in 1865 out of moral considerations, even though she was a mature 20-year-old woman who had recently married Duke Carl Theodor, a relative of King Ludwig II of Bavaria.

Pushing to the limit musically

Wagner's need to present unquenchable yearning and sexual passion in a realistic way, and at the same time to set himself the difficult task of giving some idea in music of the existential void threatening to engulf Tristan and Isolde if they are condemned to live and suffer the torment of love, led him to widen the scope of his musical resources so drastically that Tristan almost inevitably became one of the most important and revered musical works of the 19th and early 20th centuries. His second wife Cosima wrote in her famous diaries that he spoke of his need at the time of writing Tristan "to push himself to the limit musically" (11 December, 1878), strongly implying that his need to escape the constricted leitmotiv system of the Ring led him to write it in the first place. The expansion of harmonic possibilities in the very first chord of the Prelude (this so-called Tristan chord is by far the most widely analyzed collection of four notes in the entire history of Western music) plus the sheer freedom and invention in the handling of individual chromatic lines all mean that it is quite justifiable to speak of the music of the opera as a harbinger of the new music of the 20th century.

Composers have frequently acknowledged and parodied the modernist ambition of Tristan by using its opening phrase in their own works, including Wagner himself, who was the first to cite it with irony. In Act III of Die Meistersinger von Nürnberg, the stage work he wrote after Tristan, we hear the chord and the harmonic progression around it at Hans Sachs's words to Eva: "My child, I know a sad tale of Tristan and Isolde" (Mein Kind, von Tristan und Isolde kenn' ich ein traurig Stück). The historical Sachs of the 16th century did write a piece about Tristan and Isolde, though the point the character in the opera is really making is that he feels the same way as the cuckolded King Marke in Wagner's Tristan - an older man unable to possess a younger woman (Eva) and forced to give her up to a favoured younger colleague. The harmonic progression sounds startlingly like it does in Tristan, except that the hints of resignation in the dramatic situation this time suggest a rather different purpose, almost as if Wagner were saying "it will surely be hard for music to go any further than I have already taken it in Tristan." And if proof were needed, all we need do is to glance at one of the many letters Wagner wrote to Mathilde Wesendonck in the aftermath of Tristan, where in similar resigned tones he suggested that he had already reached an outside limit. "In a certain, very deep sense," he wrote at one point, "all I can do now is to repeat myself with new works" (2 May, 1860).

Most composers of any stature after Wagner disagreed entirely, including Alban Berg, who invented perhaps the subtlest, most fascinating quotation and transformation of the Tristan opening in the final movement of his Lyric Suite for string quartet - fascinating because it recalls both the musical avant-garde aspect of Tristan and its dangerous erotic reasoning. Berg blended the chord into a highly structured movement based on his idiosyncratic application of Arnold Schoenberg's serial method, and in such a way that it can be explained, as Berg himself pointed out, in terms of the working of the 12-note row. At the same time - a fact Berg did not want to be made public - it was intended as part of a secret programme referring to his affair with Hanna Fuchs-Robettin, the sister of Franz Werfel and wife of a rich industrialist. The parallel with Wagner's infatuation with Mathilde Wesendonck, the wife of a wealthy businessman, is obviously not a coincidence.

The voluptuousness of hell

Another irony of the Wagner citation in the Lyric Suite, which was almost certainly not lost on Berg, was that Schoenberg had invented "the dry mathematics of the atonal system" (as one of his critics put it) among other things precisely in order to escape the sensual and intellectual force of Tristan. Indeed, an entire generation of musicians and literary figures had succumbed to that force and proved to be unable or unwilling to free themselves from it. Friedrich Nietzsche, even after he had turned against Wagner, called Tristan the "voluptuousness of hell", adding that "the world is poor for him who has never been sick enough" to experience it. The seemingly perverse inversion of conventional morality was merely Nietzsche's way of summing up the extremes of aestheticism and décadence (he used the French word deliberately) with Tristan at its centre, which towards the end of the 19th century had long since replaced official religion as the focus of intellectual and subjective experience.

And into the first two decades of the 20th century the cult continued to flourish, this time with works like Richard Strauss's Elektra (first performed in 1909) and Schoenberg's Erwartung (composed in 1909). Both one-act operas are daring experiments in, respectively, advanced tonality and non-serial atonality that undoubtedly have their origin in the bold harmonic world of Tristan. But they also intensify the more scandalous traits of Wagner's opera that Nietzsche likened to the effects of a drug: the dissolving of worldly boundaries, the celebration of the dark corners of human existence, the retreat into sickness, and the death-devoted rebellion against existence itself, whether in the public or private spheres. These are the aspects of Tristan reflected in Strauss's and Schoenberg's operas, both of whose protagonists, like Isolde, are women searching frantically, often in dream-like states of being, for ways out of the world into a condition of untrammeled subjectivity or, as some would say, pure insanity.

If Schoenberg managed to "escape" the tentacles of Wagner's Tristan in his more astringent serial works, Strauss remained faithful to its legacy to the end of his life. On his death-bed, he is said to have uttered the phrase "Greet the world for me" (Grüß' mir die Welt) in the presence of his friend and colleague, the opera producer Rudolf Hartmann. Neither of them could remember the exact origin of the words: they were something so familiar, so ingrained in Strauss' being that it was no longer possible without conscious effort to identify where they had come from. The line is Isolde's to her confidante Brangäne in the first act of Tristan where she thinks she is going to die after taking the "death" potion with Tristan. Unknown to Isolde, Brangäne then exchanges the phials and gives Tristan and Isolde the "love" potion instead, with the result that they live on for another two acts to sing some of the most astonishing music ever written for the operatic stage.

Whether Strauss was aware of it or not, the irony of his less-than-exact recollection of where Isolde's words had come from was that he, too, had lived on after a point of crisis, which this time was the cultural pessimism that had begun to descend on many leading artists just before the outbreak of the first world war. And he had lived on to create music that remained faithful to the exquisite passions of Tristan, its yearnings, melancholies, lusts for sensuality, and last but not least the graphic presentations of sexually-driven emotion that we find, for example, in the last act of Arabella, his final collaboration with Hugo von Hofmannsthal.

At the time of Strauss's death in 1949, however, the radical, nihilistic daring at the core of Tristan that had continued to fascinate the long 19th century right up to 1914 was, like his remembrance of Isolde's words, only a distant memory - and indeed perhaps it always had been. Referring to the dying embers of his life, Strauss told his daughter-in-law at the time: "I composed it all in Death and Transfiguration (Tod und Verklärung) 60 years ago. This is just like that." It was yet another reference on his death-bed to Tristan, albeit an indirect one in the sense that it referred to a symphonic poem he had composed many years before in 1888/9 that had been strongly influenced not only by the symphonic ambition behind Wagner's opera, but also by the metaphysics of death and transfiguration enshrined in Isolde's final soliloquy at its end. Highly original as the symphonic poem is, however, not even this music could ever match the original intensity and sheer musical daring of Wagner's opera, as Strauss himself knew.

At almost exactly the same time Death and Transfiguration was being composed, Nietzsche announced in Ecce Homo (1888) that "I still today seek a work of a dangerous fascination, of a sweet and shuddery infinity equal to that of Tristan." Indeed, since Tristan was first put before its astonished audiences in Munich in 1865 nothing quite like it has ever been heard again, despite the many distinguished composers who have tried to emulate it. To this day it is a work about which we can safely say that without it Western music since the 19th century would have taken a different course. Not even Wagner's most implacable enemies could afford to ignore it. But neither have his many friends, who prepared the way for what we understand today as modern music, ever been able to recapture entirely the power of its radical spirit.

John Deathridge is King Edward Professor of Music at King's College London. He is a former President of the Royal Musical Association and author of Wagner Beyond Good and Evil (University of California Press, 2008). This article was originally published in Glyndebourne 2007 Festival programme and is reproduced with kind permission.