After Richard Wagner had been working on the Ring Cycle for about ten years, he took a break for two years to write Tristan und Isolde. With Tristan, he did something that seems unbeleivable: he took the plot of the last opera of his unfinished magnum opus (the libretto was finished, but it would take him 15 more years to complete the music) and he ripped it off.
What he repeated was the love story between Siegfried and Brunnhilde. In both operas, the man is a great hero, and the woman is equally heroic: Brunnhilde the goddess warrior queen, Isolde the Irish healer princess. Both women are proud and regal, with high status. Yet in both stories the hero gives the woman to another man, with the result that the woman is humiliated and brought to the brink of sexual subjugation. In both stories, a love potion deprives the hero of free will. In both stories, the hero is slain and the woman chooses to follow him in death.
In the Ring Cycle Wagner wrote a libretto that is the equal of the best of Shakespeare, and it is an enormous, complicated epic. Tristan und Isolde is the opposite: a splendid opera, but despite its length it is a very simple story. It has just two of themes, and they are hammered home with a heavy hand. Those themes are light/day (worldly ambition, falsity) and night (sex, death, the womb). The two extremes suggest (but don't quite admit to having) religious overtones.
In a libretto that is barely 10,000 words (in the English translation), there is a heavy repetition of the day/night themes. “Day” appears 63 times; for example, envious day, importunate daylight, spiteful day, the noonday sun of worldly fame, slave of day, day’s false glare, day’s deceiving light, spiteful day, the lies of daylight honour and fame, day’s empty fancies, lying day, phantoms of day, accursed day, night casts me back to day so that the sun can forever feast its sight upon my suffering.
Night appears 37 times. For example, night of love, the splendor of spreading night, chaste night, the noble sway of night, the wondrous realm of night, holy night, sweet night, exalted night, the dark land of night from which my mother sent me forth, universal night.
Towards the end of the opera, Wagner equates night with the ocean, culminating in the stunning words that Isolde sings to end the opera and her life: "In the surging swell, in the ringing sound, in the vast wave of the world's breath – to drown, to sink unconscious – supreme bliss!"
I can’t shake the feeling that Wagner was tugging at a thread of something important to him that was deep in his subconscious, and that Tristan was another attempt to get at it. I suspect that when he finished the libretto for the Ring Cycle he felt that there was something unexplored in that plot line and he couldn't let it go.
Perhaps the key to why Wagner covered the same ground can be found in the differences.
In both works Wagner created a woman stronger and prouder (arguably) than any female character created before, and then he dragged her to the gutter, only to have her take her own life out of love for the man who dragged her down. But they are very different: Brunnhilde is a virgin warrior goddess, while Isolde is a worldly woman, previously betrothed to her true love, and she's a healer who is unable to fight (she has two opportunities to avenge her lover by stabbing Tristan but is unable to).
In both works the man loses his free will after being duped into drinking a love potion. And in both works the woman's free will is taken from her when she is forced to marry a man she doesn't want. (In fact, in the Ring Cycle there are several women who are forced into slave-marriages.) But the context is quite different. Siegfried is totally duped to drink a love potion that wipes his memory of his love for Brunnhilde; Tristan thinks he is drinking poison, and is fully in agreement with the effects of the love potion. On the other hand, Isolde hates Tristan before being duped into drinking the love potion, while Brunnhilde chooses her unborn nephew as the man she will be enslaved to.
Another difference is that in Tristan und Isolde, Tristan sells her out before he loses his free will, and possibly before he loves her. It isn't explained why he does it, but it's clear that he has complicated feelings for her. Before the story starts, he killed her fiance in battle and sent her his head in a box. Shortly after, dying of a wound from the same fiance, he was brought to her and healed. In giving her to the king as a bride he is disinheriting himself, since he had been the king's heir. There is no indication that he knew how heinous she finds the prospect of marrying the old king - heinous to the point that she planned to kill herself.
When Isolde gives Tristan a cup and bids him drink to their reconciliation, he knows that she has put poison in the cup, and yet he drinks it happily - as does she. (As he drinks, Tristan says, "balm for endless grief, oblivion's kindly draught, I drink thee without flinching!") Only afterwards do they discover that her maid has replaced the poison with a love potion. As Isolde describes it later, "I presumptuously took death's work into my hands: the goddess of love snatched it from my grasp. She took me, death-consecrated..."
I don't understand what made this idea so powerful to Wagner, what pulled at him to create two giant operatic works about it. It's the sort of question you should ask yourself before falling asleep, hoping to find inspiration in dreaming. I do feel quite certains that most analysis of Tristan und Isolde is wrong-headed, even though it's based on letters Wagner wrote to friends at the time: that he was lamenting his unhappy marriage, exulting his mistress, and so on. Even if Wagner thought that that was what he was writing about, he was wrong.
My current thinking is that it is the subjugation and suicide of the heroine that is central. Wagner had a thing about women who are forced to be wife-slaves, but in Brunnhilde and Isolde we see a wife-slave who still loves so strongly that she kills herself when her betrayor-lover is slain. Wagner may have been dissatisfied with his creation of Brunnhilde because it was initially her father, and not Siegfried, who set her up to be subjugated. Plus, Siegfried was in a sense blameless because his actions happened while he was drugged. In Isolde, Wagner refined the subjugation: Tristan did it to her of his own free will, prior to his drinking the love potion. Isolde committed suicide for the man who tried to force her into marriage to an old man who was her country's enemy - an idea so heinous that her mother sent her off with love potion and poison in case it was too much to bear. The central image is a woman who is proud and regal who is humiliated and subjugated, but still loves her humiliator to the death.
One could think of Wagner's motivations as being enlightened or perverted. Or more probably, they are both.