Joan Bakewell: I never thought I'd ask this: can torture be justified?

It's a question I never thought I'd be asked to consider: does torture work? I have been brought up to believe in the Geneva Conventions setting out the rules for war, and that any transgression of those rules is a breach of international law. The Nuremberg trials applied those conventions and the International Criminal Court, set up in July 2002, took them as a given.

But investigative journalists have long known what is now to be broadcast, that for decades the American government, in breach, at the very least, of the spirit of these conventions, has been subcontracting its policy of torture to countries around the world. The process is known as rendition, and the major American film of the same name, released today, opens up the whole can of worms to a worldwide public.

The film is a big mainstream Hollywood thriller laced with fancy names. To many stars, the story came as a surprise. Reese Witherspoon's first reaction was "pretty much disbelief that it was happening"; But the plot is based on a true story. In September 2002 a Syrian-born, Canadian engineer, Maher Arar, was returning home via the USA when he was snatched at JFK Airport, taken to Syria where he endured months of torture.

In the film he becomes the Egyptian-born Anwar El- Ibrahimi, snatched at Washington airport and taken to North Africa. The film's good guy is a CIA man with doubts, Douglas Freeman (geddit?) sent out on his first assignment in North Africa to observe a typical interrogation. He can't stomach the brutality, agonises over whether it is effective or not, and finally makes his move. We're meant to root for him all the way through his moral dilemma. Eventually adventure takes our attention away from the moral issues and, yes, after thrills and spills galore, the good guys come out safe and sound. No mention of the ensuing trauma for the innocent torture victim.

In February 2004 President Bush denied that rendition was happening at all. He said: "Torture is never acceptable, nor do we hand over people to countries that do torture." In December that year Condoleezza Rice set out a different agenda: "Captured terrorists do not fit easily into traditional systems of military or criminal justice ... we need to adapt." In September 2006 Bush admitted that 14 men in CIA custody had been transferred to Guantanamo: Amnesty International took this as his acknowledgement that rendition does indeed happen. Amnesty's response to the new film has been to remind everyone yet again that prisoners taken in the "war on terror" are still missing and still being held in secret.

On the day of the film's launch The Medical Foundation for the Care of Victims of Torture publish their annual review, in which John McCarthy – five years a hostage in Lebanon – deplored the use of torture as entertainment. He cites particularly Fox's television thriller 24, but also mentioning James Bond's jokiness in Casino Royale as his genitals are beaten to pulp. Daniel Craig, you'll remember, recovered with easy insouciance from his ordeal. Rendition isn't like that. It must surely provoke discussion wherever it is seen. There is much to discuss. It tries as the producer says, not to be "too preachy". In fact it sets out to be balanced.

That means putting the case for torture as well as against. The case in favour of rendition goes like this: "If there's a bomb about to detonate and destroy an entire city and there's an individual who might have information, then you have the right, even the duty to get that information by whatever means you can, no questions asked. In the film the CIA's head of terrorism, played with icy single-mindedness by Meryl Streep, claims that "there are thousands alive in London today who wouldn't be...". The argument hangs fire. There are plenty of obvious flaws in such thinking – the question of who is picked up as a subject, on whose tip-off and why; the question of whether the tortured will deliver useful intelligence or just babble anything the torturer wants to hear; above all whether in using such techniques we abandon those principles and values developed over centuries that give us some right to claim to be a civilisation worth defending.

Information about such extreme interrogation techniques is available from the British army experience in Northern Ireland. It is now conceded by people that were there that it did little good, recruiting many more to the IRA side, alienating the general populace from the British and yielding no intelligence that was thoroughly reliable.

But at a discussion following a preview of Rendition, both James Rubin, former aide to President Clinton who introduced rendition long before 2007, and the international lawyer Philippe Sands agreed that we are now in uncharted territory. Time was when the hydrogen bomb was the biggest threat to humanity. No one thought there could be worse weaponry available to mankind. Espionage consisted of seeking out and frustrating such potential attacks.

Then the suicide bomber came along, operating beyond the reach of traditional criminal and military pursuit. This is the new context in which we have to protect our populations. With weapons strapped to their bodies, terrorists can infiltrate targets in ways undreamed of by tradition military strategies. To counter their power means rethinking how we reach them before they reach us. Rendition will make you wonder how we might do that.

joan.bakewell@virgin.net