I loved "Blood Diamond," until I didn't.

This gripping, sophisticated thriller, starring Leonardo DiCaprio, Djimon Hounsou and Jennifer Connelly, is an extraordinary achievement in American cinema, one that combines visceral thrills, high production values and morally serious ideas. In this swiftly moving tale of geopolitical suspense set in Sierra Leone during the 1999 civil war, DiCaprio plays a diamond smuggler named Danny Archer, who routinely buys gems from bloodthirsty rebels to sell them to middlemen for European dealers. Hounsou plays Solomon Vandy, a man kidnapped by the rebels, who loses his family and is forced to work in one of the guerrillas' mines. When he finds a huge pink diamond, he manages to hide it just before the camp is broken up by government troops. Archer and Vandy eventually join forces to find the diamond, each for his own reasons.

Director Edward Zwick ("Glory," "The Last Samurai") packs an incredible amount of information into the first two hours of "Blood Diamond," which was filmed in South Africa and captures with vivid, kinetic energy the beauty of the African countryside, the cultural mash-ups of its cities and the rank horror of its myriad bloody wars, here put firmly in the context of 300 years of colonization and exploitation. ("Let's hope they don't discover oil here," one villager says. "Then we'd have real problems.")

As Archer and Vandy make their way out of the lawless powder keg of Freetown -- with the assistance of an idealistic journalist played by Connelly -- Zwick strikes a terrific balance between Indiana Jones-style adventure and an ongoing dialogue about the uneasy and largely opaque politics of globalization. How many women would want that diamond engagement ring, Connelly's character asks at one point, if they knew someone had lost a hand getting it?

A good question, among many raised in "Blood Diamond," which features impressive performances from its three stars, especially DiCaprio. Between this movie and "The Departed," the 32-year-old actor seems to have enjoyed a growth spurt this year; his baby face has given way to a new fullness and gravitas, and he wears the new heft well.

As engaging as the three leads are, what's most impressive about the movie is a subplot having to do with Vandy's son Dia, who is kidnapped in the same raid as his father, but is turned into a child soldier by a rapacious rebel leader. Of all the scenes of war, mayhem and suffering in "Blood Diamond" (and there are many), the sequences of children being turned into psychopathic killers are the most disquieting, haunting and potent.

Indeed, "Blood Diamond" has so much to recommend it that one hesitates to bring up quibbles. But problems do arise, all in the last half-hour, and all having to do with Hollywood conventions, whether it's a final overlong, explosive battle scene, a hero's send-off or the vaguely offensive conclusion in which Vandy is portrayed as the voiceless moral conscience of the film (yet again, our tour guides of Africa have been white, with the black character reduced to noble but passive symbol).

That "Blood Diamond" has hitherto so adroitly blended genuine action, suspense and moral complexity only makes disappointment the greater when the film falters. Still, it gets far more right than it does wrong, and its most indelible images -- of the chaotic human landscape of war, of Africa in its enduring verdant beauty, of those tragic children so obscenely manipulated -- will burn in the viewer's consciousness long after the less edifying moments have faded.

Blood Diamond(135 minutes, at area theaters) is rated R for strong violence and profanity.