The trouble with Hollywood is that when it gets hold of a serious subject, it feels impelled to dress its argument up either as an action movie or a love story, in case we all get bored. It's understandable but frustrating.

Ed Zwick's film, set in the battle-torn Sierra Leone of the Nineties, is about the trade in "blood diamonds", sold on the market after discovery by put-upon African workers so that war and mayhem can continue to be financed.

Its central character is Danny, Leonardo DiCaprio's Zimbabwean ex-mercenary. He's presented to us as a man without scruples, making a convenient bargain with Djimon Hounsou's excellently played villager who has been taken from his family and forced to work in the diamond fields.

The bargain is that DiCaprio will help him find his wife and children, one of whom has been forced by the rebels to become a child soldier, if Hounsou will lead him to the rare pink diamond he has found and hidden in the fields.

So far, so good. Eduardo Serra, the cinematographer who made The Girl With The Pearl Earring look like a painting by a Dutch master, this time portrays Sierra Leone as a beautiful country being destroyed by a civil war of appalling ferocity, in which children are forced to play their part and limbs are lopped off without compunction.

And Zwick, assembling his cast largely from Africans, watches a number of convincing if sometimes horrendous cameos.

This, one thinks at one point, is the US version of The Constant Gardener. Then we get Jennifer Connelly as an impossibly pretty American journalist trying to persuade DiCaprio to mend his ways, and we see the Zimbabwean begin to see the light.

In the end, he's a changed man and in love. But the film is also changed, and not for the better.

While DiCaprio, after playing for Scorsese in his last two films, has become a much better actor, and possibly deserves his Academy nomination, even if his accent does stray, there is no doubt that Zwick over-simplifies the Sierra Leone conflict and makes the blood diamond conspiracy seem more black and white than it is.

Perhaps he had to, yet he seems incapable of editing down a film that stretches far too broadly at 143 minutes.

What remains is a comparatively honest effort from Hollywood but one fatally weakened by the familiar scent of compromise in its final reels.

The plus point is that it has caused those who might be thought guilty of the blood diamond trade to issue all sorts of disclaimers. That, at least, is a strong point in its favour.