Who decides what happens here?

My son and I enjoyed watching the movie, Green Zone, which concerns an American solider in Iraq searching vainly for Weapons of Mass Destruction (WMD) after the invasion. He realises that the existence of WMD's was based on dubious intelligence which was concocted by the US government to justify the invasion. Although a "fictional" Hollywood account it does make a strong political statement and resonate with some real events.
For example, the journalist Lawrie Dayne in the movie seems to correspond to Judith Miller who wrote front page stories for the New York Times claiming there were WMD's. However, it later became clear her stories were based on dubious sources which she did not properly check.
The movie highlights the issue of de-Ba'athification of the Iraqi government [i.e., not including in the government anyone who had an official association with the Baath party of Saadam Hussein]. Many Western analysts now consider this was a significant mistake which should not be made again in Libya.
Wikipedia notes the most important line in the script:

James Denselow, writing for The Guardian, praises the film's portrayal of the conflict, saying "ultimately what gives the film its credibility is that it avoids any simplistic idea that Iraq could have simply been 'got right'. Indeed Miller's [the American soldier] vision of exposing the WMD conspiracy and the CIA's plan to keep the Iraqi army is undermined by the film's wildcard – a nationalist Shia war veteran who turns the plot on its head before delivering the killer line to the Americans when he tells them: 'It is not for you to decide what happens here [in this country].'"^[41]