How Rwanda and 9/11 moved me to write for Leonard Bernstein

ernstein was never entirely comfortable with his own text, though. "His words were elegant but he felt they seemed weak and needed a stronger spine," says Pisar.

The composer implored him to create something new based on his experiences of suffering and rebirth. But Pisar resisted, saying that he "could not write poetry at the level of your music". Though Bernstein died in 1990, Pisar's wife, Judith, and others, "continued to nag". Eventually Pisar, who in 1980 had written a powerful autobiography called Blood and Hope, relented.

So what changed his mind? After seeing new tragedy unfold in Cambodia, Bosnia, Rwanda and Somalia, Pisar wondered whether "the unthinkable still remains possible". Then with 9/11, he saw perpetrators who were not pagans like Hitler or Stalin, but zealots prepared to kill in God's name. He decided to write a text that would be Jewish and universal, personal and public - a warning against our capacity for evil, yet also a testimony to humanity's intimate, at times doubting, relationship with an ever-present God. It was not an easy task, touching on the old question: "Where was God at Auschwitz?"

"I didn't want to re-open my quarrel with the Almighty. I was continually floating between faith and doubt, asking: Do I believe or don't I believe?" Pisar explains. "I also wondered: ‘Should I do it in the voice of the little one, myself at 10, thrown into the pit of hell? Or should it be the voice of the sophisticated, worldly adult? Further, would my fellow survivors understand if I came out as an atheist, or would modern friends understand what had hit me if I sounded like a born-again believer?"

Ultimately Pisar "let the child speak"; and as a result, he has, he says, "attained a universal feeling precisely because it is so personal". In an audacious dialogue with God he reminds the deity that those who died did so with his name on their lips. Thus, they are bound to Him and He to them. "It's a complex concept yet also quite a basic one," he notes, adding: "Mine is a layman's kaddish, dedicated to your tormented children, Jews, Muslims, Christians, believers and non-believers."

Pisar's version of Kaddish was premiered in 2003, yet his role began nearly 50 years ago when he met Bernstein via his own wife, Judith, an American musicologist. The genesis of the work was arduous.

Bernstein conceived Kaddish in 1955 yet took eight years to finish it. Its inaugural concert in Tel Aviv in 1963 occurred just weeks after the slaying of President Kennedy, to whose memory it was dedicated. (In 1960, Pisar was made a senior presidential foreign policy advisor to Kennedy, whose first act in office was to grant him US citizenship).

The tale of "Pisar's Kaddish" really started in April 1945 when Allied troops liberated the then 15-year-old. Pisar had escaped from a death march out of Dachau, having earlier witnessed the murder of both parents and been incarcerated in Auschwitz. He was persuaded by an aunt to leave for Australia and plunged himself into English studies - "it is my adopted mother tongue, even though I find myself returning to Litvak Yiddish", he jokes.

Happily mentors encouraged him and helped him "undo the Holocaust, even at an infinitesimal level, particle by particle". Pisar eventually took a PhD at Harvard and for decades crucially helped broker US-Soviet talks during the Cold War. He and Judith built a happy family with their four children.

Once considered an orchestral white elephant, Kaddish is now being recognised as one of the most powerful pieces of 20th-century music. To Pisar it "expresses a deep affection for the Almighty. Yes, I was angry with him as a kid, yet now I realise that, illusion or reality, we are stuck with him."

Kaddish is performed at the Barbican, London EC2 on Tuesday October 28 at 7.30pm. Tickets on 020 7638 8891. Proceeds go to the Aegis Trust and the Holocaust Centre in Nottingham