MUSICIANS IN Chile are, by and large, a left-wing lot, but they grudgingly admit a right-wing debt: it's thanks to Pinochet that they now have world-class opera.

Why do tyrants love music? Next week's Sound Stories (Radio 3) dwells on the converse, showing how composers have been inspired by Nero, Montezuma, Alexander the Great and Peter the Great - but the final programme poses this same question. Stalin may have given Shostakovich a hard time, but he was so devoted to Mozart's 23rd Piano Concerto that he demanded a special recording to be made of it. This recording was apparently found on his turntable following his death.

Now consider Lenin's verdict on Beethoven's Appassionata: "Astonishing, superhuman music... What miracles people can do... But I can't listen often to music. It affects my nerves, makes me want to say kind stupidities and pat the heads of people who, living in this dirty hell, can create such beauty." But you must not pat people's heads, he goes on; you have to beat them mercilessly. Lenin's response to Beethoven reflects the terror of the addict when faced with the drug that threatens to destroy him.

Since music is the creation of an ideal order, no one should be surprised that it attracts those with a penchant for control. But the connection with cruelty is more mysterious, and the Nazis are the locus classicus here. Hitler may have loved Wagner for ideological reasons, but many of his henchmen were out-and-out music fanatics. Goebbels was obsessed with opera as much as with films; Dr Mengele liked his Auschwitz victims to serenade him with Schumann; Baldur von Schirach, the Gauleiter of Vienna, was a musician as well as an opera buff - after the day job persecuting Jews, music was his nocturnal solace.

This weekend sees the opening of London's annual Jewish Music Festival, whose events include performances of works that the Nazis just as passionately did not like. Stalin proscribed music with "formalist tendencies": the Nazis proscribed entartete ("degenerate") music, embracing everything from jazz (by "degenerate" blacks) to atonal modernism (by "degenerate" Jews). Schoenberg, Berg, Webern, Hindemith and Weill acquired that label.

While this festival celebrates the music of the Dutch-Jewish composers who perished in the camps, Wexford is celebrating the first staging for 60 years of an opera by one of the most prominent Czech victims. A recording of the work has, coincidentally, been released by Decca. Pavel Haas was Janacek's most promising student, and his opera Sarlatan is full of Janacekian echoes. It's a minor work, but it forms part of a major series of recordings entitled, with massive irony, Entartete Musik. The composers represented in the series formed a strand of creativity that might (had it not been snuffed out in the gas chambers) have helped prevent Western music's catastrophic lurch into intellectualised aridity.

Most of these composers were incarcerated in the Czech town of Terezin. This was a concentration camp with a difference, in that the Nazis wanted to present it to the world as a "model" ghetto, whose inhabitants were happy and fulfilled. It was the Nazis' good fortune that among their prisoners should have been a large number of writers, painters and composers. Haas was one of these, and his coeval Viktor Ullmann another, and Ullmann's bequest to posterity was a one-act opera of exquisite perfection. Der Kaiser von Atlantis is an allegorical tale of tyranny overthrown, but its contemporary references were unmistakable for the SS officers, who aborted it in rehearsal. The libretto returns again and again to the themes of imprisonment and exile, the comfort of death and the indomitability of the spirit. "Come Death, honoured guest, enter the chamber of our hearts," sing the protagonists at the close, but the tune they sing it to is a skewed variation on Bach's "Ein feste Burg", a Protestant hymn much favoured by their oppressors.

Mecklenburgh Opera staged a small production of this heart-rending work three years ago. It's time one of our big companies had the imagination to do it. Meanwhile we have the excellent Decca recording.

POETRY, SAID Auden, makes nothing happen. Music, said Stravinsky at around the same time, "is essentially powerless to express anything at all". In other words, art for art's sake. But Plato knew the true score: for him, music was an inescapably moral force - whether for good or evil - and it followed that certain kinds of music should be promoted, and others banned.

On 16 November at the Union Chapel, Islington, that excellent magazine Index on Censorship is hosting a benefit concert on this theme. An "Evening of Banned Music" promises to be a piquant occasion, with a gamey musical mix and Simon Rattle taking a turn as piano accompanist. But that word "banned" is somewhat loosely employed; "cold-shouldered" would be a more accurate description of several of the works in question.

Ever heard of a prolific British opera composer called Alan Bush? If not, it's probably thanks to his blacklisting by the BBC throughout the Cold War, purely because he was a Communist. The programme also includes Messaien's Quatuor Pour La Fin du Temps - which was composed and performed in a prisoner-of-war camp - plus a pianola extravaganza by Conlon Nancarrow, who decamped from McCarthyite America in disgust.

But in a sense Stravinsky was right: music's political subversiveness resides not in what it expresses, but in its associations. Another of Index's selected works is Guridi's Cant dels ocells, once banned by Franco as a symbol of Catalan resistance, but in 1992 the theme tune for the Olympics.

It is a shame that Index on Censorship couldn't have included something by Victor Jara, whose music - banned on pain of death after his murder in 1973 - will pursue the old codger in the London Clinic from this world to the next.

The Jewish Music Festival runs from 1 to 29 November, with more than 50 events at various London venues (0181-909 2445). Details of Index on Censorship's `Evening of Banned Music' are available on 0171-439 1783