The star and director of the last two “Bourne” movies are back and much is the same — the gritty, intimate, documentary feel, the sense of peril and dynamic staging of action, the able but conflicted leading man. But there is an important difference. “Bourne” is based on a series of novels, but “The Green Zone” is based on a non-fiction book, Imperial Life in the Emerald City by former Washington Post Baghdad bureau chief Rajiv Chandrasekaran, about the failed search for weapons of mass destruction in post-Mission Accomplished Iraq.

The “Bourne” movies were more than the usual slick spy story. Bourne was spying on his own past and what was revealed did not match real-life events but it resonated with them, giving the films some extra heft. “The Green Zone,” however, bases the story in recent events. It tweaks the names and some of the circumstances of the main characters, but not enough to establish a separate, consistent reality, just enough to be distracting. Audiences will look at the Wall Street Journal reporter played by Amy Ryan and stop to whisper, “Is she supposed to be Judy Miller? Is there a reason that a different character’s name is Miller? And who is that other guy supposed to be?” Those who are up on all of the details of the Iraqi war will be distracted by what is missing. Those who are not will be distracted by what is included.

As Damon and his men chase through crumbling buildings on blown-up streets, chasing and being chased, we see that all of their crack training and cutting-edge technology are no match for a situation that does not meet any previous military definitions or capacities. There are no foxholes or battle lines. Like the Light Brigade, they are expected to charge forward, “theirs not to reason why, theirs but to do & die.” But when a Chief Warrant Officer (Damon) finds that he is repeatedly risking his life to retrieve weapons of mass destruction that do not exist, he wants to find out why the intel is so consistently unreliable. And then, when no one else seems to care about that, he wants to find out why. Shock and Awe seems to have deteriorated quickly into a quagmire.

His quest takes him though a crumbling palace, chandeliers incongruously shoved aside, to an even more surreal location in the American compound, with girls lounging in bikinis by a pool, being served pizza and beer. He meets a local with a prosthetic leg (Khalid Abdalla, excellent as “Freddy”), who leads him to the man who is the Jack of Clubs in the war criminal deck of cards. But it turns out that his mission is not what he had thought. “Democracy is messy,” a Pentagon official (Greg Kinnear) tells him. “We’re here to do a job and get home safe,” another soldier says. “I thought we were all on the same side,” the Chief Warrant Officer tells the CIA representative (Brendan Gleeson). “Don’t be naive,” he responds. It turns out hardly anyone is on the same side as anyone else. Both sides have splintered into factions with shifting loyalties and murky motives. And the wall of the prison where Iraqis are being tortured says, “Honor Bound to Serve Freedom.”

But this script’s attempts to be intricate underscore how much it simplifies the reality, especially with a gesture at the end that is supposed to be cathartic but instead just makes us question the reliability of everything we’ve seen. Over-simplified and under-played, this movie wants to be more than the fictional Bourne series but ends up being less. I’m betting that this was a studio-imposed effort to make the film more marketable after a series of disappointing box office returns for Iraqi war movies. Some day, maybe, there will be a director’s cut that will recognize that like democracy, some movies need to be messy, too.