STEVEN Spielberg originally wanted to make the film 12 years ago. He bought the last surviving script for Orson Welles's radio version of The War of the Worlds, and was immediately hooked. He had read the original novel several times while at college, but it was the script that really brought the movie possibilities home.

"It was amazing, I guess you could say a distillation of the novel. I said, 'Oh man, this would make an amazing movie'," he says.

This was the version performed by Welles's Mercury Theatre which caused nationwide panic when it was broadcast in America in 1938. Its depiction of a Martian invasion was so realistic that when it finished, with a reporter gasping 'This is the end now', as he was apparently asphyxiated, listeners caused traffic jams as they attempted to flee their cities and phone lines went down under the weight of terrified calls.

The script, owned by Howard Koch who co-wrote the play with Welles, was the only one not at the theatre when police arrived and destroyed every copy they could find.

But Spielberg, the Oscar-winning director of Saving Private Ryan and Schindler's List, put his invasion plans on ice when he realised he had been beaten to the punch by director Roland Emmerich.

"When Independence Day came out I said, 'Well, maybe I won't make it', because they kind of picked the bones of that and put me off for a while," Spielberg says.

It was not until he was looking for another project with Tom Cruise, who starred in the director's Minority Report, that his mind came back to War of the Worlds. The result is a picture which, at a rumoured $200m, is one of the most expensive ever made and, when it opens in cinemas on Friday, is tipped to be the summer's biggest blockbuster.

Cruise plays Ray Ferrier, a crane operator on the Brooklyn docks, who is caught up in the alien takeover. While the world is engaged in a struggle to the end, Ferrier's priorities are his ex-wife, played by Lord of the Rings' Miranda Otto, and children Justin Chatwin and 11-year-old Dakota Fanning, starring in her 12th film.

Unlike Welles's version, which stayed faithful to the novel by his near-namesake HG Wells in having the aliens from Mars, Spielberg's invaders are from further afield. But after feverish speculation, and following tight secrecy, he has confirmed that the tripods are still there.

For Spielberg, it represented a sharp contrast with his previous treatment of aliens, as benevolent and inquisitive visitors, in both Close Encounters of the Third Kind and ET. And it was the changed international climate which provided the context.

"I'm just an equal opportunities director," he says. "I gave the nice aliens a couple of shots and now I'm gonna try my hand at the worst kind - the kind that's just bent on ending civilisation as we know it, and beginning their own.

"I think in the shadow of 9/11 it began to make more sense to me, that it could be a tremendous emotional story as well as a very entertaining one, and have some kind of current relevance."

For Spielberg's screenwriter David Koepp, the alien invasion can be read as a metaphor for US foreign policy, with New York the battleground instead of Baghdad. And in the end the all-powerful global force is defeated by a "local insurgency".

But if Spielberg's film of invasion and global crisis has been given a new significance by the World Trade Centre attacks and subsequent invasion of Iraq, it is merely continuing a long tradition of setting alien threats against the background of international insecurity.

In Wells's original 1898 novel, the Martians land not in New York but in Woking, Surrey, from where they advance into London and demolish the city to make way for a nursery to grow the plant that keeps them alive. Their remorseless and brutal invasion was a critique of the way European empires had overrun and enslaved large areas of the globe, and their demise at the hands not of their enemy but of lowly bacteria, a reminder of the frailty of these empires.

If Orson Welles's radio version exposed the vulnerabilities of 1930s America, a 1950s film version reflected both the horrors of the Second World War, and the anxiety of the emerging Cold War and the threat of nuclear holocaust.

This 1952 film, directed by Byron Haskin and starring George Pal, as well as Ann Robinson and Gene Barry, who both have cameo roles in the Spielberg movie, included newsreel film of bombed cities to depict the devastation left by the Martian advance, and its tale of competing nuclear technologies - the Pentagon fires an atomic bomb at the alien ship, only to see it repelled by electronic defences - struck a chord with audiences coming to terms with life on the edge of nuclear war.

This version was in a similar vein to a host of 1950s science fiction which emphasised the precariousness of human existence, under threat both from alien worlds and from man-made dangers. One of the best in the latter category was Them!, a 1954 movie in which radiation from atomic bomb tests caused giant ants to breed in the New Mexico desert.

One of the clearest Cold War parables came in the 1951 classic The Day the Earth Stood Still. This Michael Rennie-starring film saw a flying saucer land in Washington, with its alien occupant on a mission to warn the human race of the danger of continuing on its warlike path. When the US military shoots the visitor, its robot begins to destroy the Earth, and it falls to a group of peace-minded scientists to stop it, by use of a password.

As the threat of nuclear war seemed to recede, movie aliens began to be seen as potential friends, separated by biology just as Russians were separated only by ideology. But the fall of the Berlin Wall liberated film-makers to once again depict life on other planets as hostile, notably in the two 1996 movies, Emmerich's Independence Day and Tim Burton's Mars Attacks!

Inspired by bubble-gum cards, Mars Attacks! parodied 1950s sci-fi, but also introduced a new pessimism, as the loathsomeness of some of the human characters introduced doubt over whether this was a race worth saving. This returns to the spirit of Wells' novel, which is not really about aliens at all, but about the self-destructiveness of its dominant race.

* The War of the Worlds opens in cinemas on Friday. For a review and interview with Tom Cruise, see Thursday's 7Days.