Certain political events irrevocably change society, but there are psychic events as well - equally portentous though harder to pinpoint - that alter human consciousness. One of these was the first performance of Wagner's Tristan und Isolde in 1865; we are still attempting to absorb or (if possible) to overcome its influence, not only on music and drama but on our emotional lives. The inquest will continue this month, when Nikolaus Lehnhoff's new production of Tristan und Isolde opens this year's Glyndebourne season while English National Opera revives David Alden's staging of the work. I predict agitated nights and enervated days for anyone attending both.
Wagner boasted, while working on the score, that Tristan would drive people mad, and indeed its febrile chromaticism, aching deferments and eventual orgasmic climax permanently jangled Europe's nerves. The disruption began with the first bars of the prelude: a wistful, wilting dissonance - the audible sign of frustration, given that the disharmony is not resolved until five hours later, in Isolde's dying utterance. That illicit chord announced the end of a tonal system that had kept the world in tune. The Tristan chord became, for modernist composers, a motto of helpless, obsessive neurosis: Schoenberg's tutor Alexander Zemlinsky quotes it in a song about a woman bedecking her hair with blood-red poppies, and it recurs in Alban Berg's Lyric Suite as a coded confession of Berg's marital infidelity.
Wagner's characters still operate according to the worshipful, courtly code imposed by the medieval romance from which they derive. Consummation is painfully delayed. Isolde has two yelping high Cs of coital exhilaration before the tryst in the dark garden, but she attains her long-awaited climax only after her partner's death; Tristan, deranged by guilt, tears the bandages off a wound that might be venereal. Wagner's modern followers had no patience with such moral compunctions. In L'age d'or, a riotously blasphemous film on which Salvador DalI and Luis Bunuel collaborated, a palm orchestra plays Isolde's sublimely abstracted "Liebestod" at an open-air concert. The hero and heroine of the film meanwhile perform the music by rutting in the dirt.
The symbolist painter Henry de Groux - who specialised in dimly mystical illustrations of Wagner - gave his mistress the score of Tristan und Isolde, inscribed with the assurance that it would teach her all she needed to know about love. Was the opera then a sex manual? If so, it provoked its imitators to morbid, self-destructive excesses. In Gabriele D'Annunzio's novel Il Trionfo della Morte, two besotted lovers act out the opera by making a suicide pact; in Thomas Mann's story "Tristan", another pair of consumptives in a sanatorium play through the score on a piano, and although the music remains inaudible, its nervous strain causes the woman to cough up blood and die. The opera's words have a malevolent power even when separated from the music: T S Eliot twice quotes the libretto in "The Waste Land", identifying the opera as a symptom of the despairing self-indulgence that lays waste to the modern world.
By 1930, Mann had decided that the score itself was a "noble malady", toxic and highly contagious. The epidemic extended even into popular culture, where the malady came to seem messier and more ignoble. In Fritz Lang's thriller The Blue Gardenia, a murderess uses the "Liebes-tod" as an accompaniment to slaughtering her lover. Bernard Herrmann's score for Hitchcock's Vertigo, quoting the opera when James Stewart makes love to the resurrected Kim Novak, supplies a clinical diagnosis of that love-death: what we watch, through the green mist that suffuses the room, is an episode of necrophilia. The "Liebestod" proves equally fatal to Joan Crawford's predatory socialite in the fraught melodrama Humoresque. Her protege, a violinist played by John Garfield, has spurned her; listening on the radio to his performance of a Tristan und Isolde Fantasie by Franz Waxman, she stalks out of her beach house and, at the moment when Isolde should be singing about the oceanic swell of extinction, she drowns herself.
The composer Constant Lambert, hoping that Romanticism had exhausted itself, remarked in 1933 with some relief that there was "no possibility of a modern Tristan", because such crazed ardour had vanished from the world.
W H Auden during the 1940s took to assuring anyone who would listen that Tristan and Isolde were really a pair of lesbians. "A man making love to a woman," he insisted, "couldn't really get into that rapturous state. He'd be thinking about something else!" The outrage was deliberate: how else could the Romantic malady be cured?
As if to prove Lambert's sternly ideological point, Graham Greene sends the characters in his novel England Made Me to a performance of the opera in Stockholm. They are preoccupied by political intrigue, and consider the music - with its "doped drink" or "soprano sorceries", and all that "fuss over irreparable love" - to be irrelevant and intrusive. That initial dissonance sounds merely wayward. "The violins," Greene comments, "began tentatively to search for something and could not find it." He unwittingly paraphrases Stravinsky's objection to the chromatic languor of the score, against which the composer had been trying to immunise his own music: Wagner's "endless melody", Stravinsky joked, was "the perpetual becoming of a music that never had any reason for starting".
Yet, once having started, it was unstoppable. All those dismissive declarations about Tristan were premature and powerless. Friedrich Nietzsche, attempting to purge himself of his earlier mania, in 1888 said that "Wagner is bad for youths; he is calamitous for women." So he is, but that accounts for his dangerous, irresistible allure. It is unlikely that crazed Wagnerites will hurl themselves into the ha-ha at Glyndebourne, but Tristan remains the ideal Romantic work of art. Its intensity is liable to induce delirium; a great performance might make you think, at least until the lights come up, that you are dying of bliss.
Tristan und Isolde is performed at Glyndebourne, East Sussex (01273 813 813) from 19 May to 4 July, and at English National Opera, London Coliseum, WC2 (020 7632 8300) from 24 May to 8 June