George Clooney has acted in Second World War movies before, with mixed success.

Terrence Malick's The Thin Red Line was a poetic and affecting account of the war in the Pacific; but Steven Soderbergh's The Good German, set in Berlin shortly after the Allied victory, was formally daring but dull as dishwater.

At least Soderbergh, who shot his drama in the style of a 1940s studio film, failed while attempting to be adventurous. Clooney's first WWII film as a director fails because it is far, far too safe. In fact, The Monuments Men is so conventional, so desperately earnest and inoffensive, that it could be one of the worst war films ever made.

It's hard to say this, because Clooney is an interesting and likeable multi-tasking superstar. Good Night, And Good Luck and The Ides Of March - in which he acted as writer, director, producer and star - were intelligent, superbly acted and handsomely mounted films. But the stuttering comedy Leatherheads showed that he is not infallible. And this new film confirms the possibility that when Clooney is not on familiar ground, he can become predictable and trite.

The film is based on the book of the same name, about the group of art historians, curators, architects and others who rescued thousands of artworks stolen by the Nazis. The Germans wanted these paintings and sculptures for themselves; as defeat loomed, they were prepared to destroy them.

When art historian Frank Stokes (Clooney) briefs President Roosevelt on the problem, we are given an early taste of the excruciation to come. "These are the greatest historical achievements known to man," says Frank sagely. "We have to make sure that the Mona Lisa is still smiling, and the statue of David is still standing."

Stokes is despatched to Europe, with a mission to find what is missing and protect what remains from Allied bombing. First he recruits his team of ageing and unfit art folk. A gaggle of stars, including Matt Damon, Bill Murray, John Goodman and Hugh Bonneville, play fictional versions of the real people. Unfortunately, we learn next to nothing about them - their areas of expertise, family lives, connections to Stokes. That such men were prepared to go to war for art is reason enough to make the film; but Clooney errs in not developing more rounded characters.

Likewise, the tone is all wrong as they reach Europe - strolling on to the Normandy beaches as though into a holiday camp, driving through the Battle of the Bulge with no-one batting an eyelid. When the reality of their inexperience does catch up with them, the pathos simply hasn't been earned.

There is some calibre on display, but it doesn't involve a monuments man. Cate Blanchett plays a French curator wrongly deemed to be a collaborator for continuing in her post as Goring "comes shopping". In fact, she is keeping a record of the art in his basket, and its secret location, in the hope of one day retrieving it. Blanchett is terrific as this brave, bitter heroine, whose confrontations with the Nazis convey a danger more real than anything we see elsewhere.

Incidentally, as Clooney's ­character recruits his team members we are reminded of the actor performing the same task in the caper movie Oceans Eleven. The Monuments Men would have been far better if it had been made as a caper movie itself, something akin to the 1970 Second World War-set comedy Kelly's Heroes, than as a respectful homage that can't be regarded as any kind of art worth fighting for.