From the day Tom Cruise put on his eye patch, squeezed into a pair of knee-high jackboots, and started working on his Nazi goosestep, pundits have been queuing to declare the Second World War thriller Valkyrie the most reckless gamble of his career. The film, a $90m (£60m) portrayal of the 20 July 1944 plot to kill Adolf Hitler, in which Cruise plays the failed assassin Claus von Stauffenberg, has been dogged by an almost comic array of problems since its inception two-and-half years ago.

Politicians tried to block filming in Germany. Relatives of Von Stauffenberg expressed disapproval at Cruise's links to Scientology. A team of extras was injured during production and launched a $11m lawsuit. Major scenes needed to be reshot, and the release date was changed more than three times.

This week, the film's trailer hit US cinemas. And as billboards began popping up around Los Angeles announcing its Boxing Day release, a new and even more shocking revelation began doing the rounds of Hollywood: against all the odds, Valkyrie may actually turn out to be rather good.

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On Saturday and Sunday, in cheerful defiance of the wild-fires that gridlocked much of southern California, Metro-Goldwyn-Mayer held its first press screenings of the film, which co-stars Bill Nighy, Kenneth Branagh and Eddie Izzard.

The result was a double triumph. Not only were a handful of industry reporters present able to scotch dark rumours about Cruise's German accent (in the event, he does not attempt one), they also gave the film almost shockingly positive reviews.

"All the buzz is that it's pretty good," said Variety's executive editor, Steven Gaydos. "Von Stauffenberg is not a typical role for Cruise, but in the event, he is a terrific actor who has surrounded himself with some of the most talented people around."

Downfall, starring Bruno Ganz, demonstrated that Hitler films can bring box-office success. Valkyrie has been test-screening since August, and is now said to be receiving 80 per cent positive responses from audiences, a surprisingly decent figure for the tale of a one-armed, one-eyed colonel who was killed for attempting to assassinate Hitler with a briefcase bomb.

Buoyed by the reaction, MGM decided to go for the highly competitive holiday release date (the film was until recently scheduled for launch on Valentine's weekend). This means it will clash with new films from Jim Carrey and Brad Pitt but gives it an outside chance of Oscar nominations.

For both Cruise and the studio, any such result would represent an extraordinary comeback. In the 20 months since Valkyrie was given the green light, the film, with its sprawling narrative that crosses several continents, has lurched from crisis to crisis. It now represents a critical test of both of their futures.

MGM plans to invest $60m marketing the film, and is in dire need of a major hit. The firm has been struggling in the credit-crunched economy to raise $650m to finance its next major slate of films, which include two instalments of The Hobbit and the next James Bond film.

Its only major successes in recent years have been the past two Bond films, over which it had partial control. The recent collapse of Merrill Lynch, which has a $500m production facility with the company, prompted MGM's chairman, Harry Sloan, to launch a review of its finances.

Yesterday, The New York Times added to the studio's woes by revealing that Mr Sloan's hunt for the new $650m production fund was "dead", quoting two bankers close to the firm blaming, "in part, the global economic crisis, but also MGM's staggering $3.7bn debt load". For Cruise, the stakes are also high. Despite being at the top of the Hollywood tree for a quarter of a century, his private life has become a magnet for negative publicity, thanks mostly to his association with Scientology.

More pressingly, Cruise has not managed a major hit since Mission Impossible III in 2006, and has been absent from the screen except for a small role in the lightweight comedy Tropic Thunder, and another small part in the war on terror flop, Lions for Lambs.

The fate of Valkyrie is not just critical to his acting career. It will also have a major impact on his standing as a film producer. It was originally approved by United Artists, the MGM unit of which Cruise and his producing partner Paula Wagner were part owners. Wagner resigned in August after the costly failure of UA's Lions for Lambs. Though Cruise still keeps an office at MGM's headquarters in Century City, Los Angeles, another flop will probably herald the end of his producing career.

Fortunately, for Cruise at least, Valkyrie is a more intelligent beast than its original "action flick" billing suggested, and was directed by the highly regarded Bryan Singer. "The first impression of a Bryan Singer meets Tom Cruise movie is that you've got X Men meets Mission Impossible," admits Singer in one of the film's new trailers. "But you may forget that I also made The Usual Suspects, or that Tom made Born on the Fourth of July."

To restore Hollywood's confidence in Cruise and MGM, Valkyrie will need to justify that billing by topping the charts in the week of its release. To actually make money, it will also need to take $150m in the US, and a similar amount when it is exported to the UK and Europe in January.

It is a stiff task. But with a following wind, pundits say it could help define the rest of Cruise's career, which, as he approaches his 47th birthday, needs a change from its stock-in-trade of boyish action heroes. "Tom Cruise has managed 25 years as one of the biggest stars in the world, but at the end of the day, as his recent cameos have suggested, here is a man pushing 50," said Steven Gaydos. "For the next 25 years, he needs to find more mature roles."

Despite all the problems that have dogged Valkyrie, in Claus von Stauffenberg, Cruise may have stumbled on the perfect alter ego to achieve just that.