One of the unpredictable things about the post 9-11 "war on terror" is the way it has inspired so much poor work - heavy handed, smug, ill-informed or even dishonest - by so many well-regarded writers, directors and actors.

From Robert Redford it prompted 2007's confused Lions for Lambs where he played a politics professor to ensure that the audience got the full benefit of his incoherent, 1960s-formed views.

Now he has directed a film that takes on the war on terror through historical analogy. The result is well-meaning but clunky period courtroom drama whose effectiveness is undermined by the filmmakers' lack of faith in the audience.

The screenplay by James Solomon, turns the characters into mere mouthpieces for points of view about the importance of giving a genuinely fair trial to even the vilest of criminals. By the end you feel like you have been subjected to a crudely one-sided civics lesson.

The ostensible subject of the film is the trial of the Confederate conspirators whose plot to decapitate the Union on the night of April 14 1865 resulted in the assassination of Abraham Lincoln. Perhaps to underline the parallels with 9/11, the film does not shrink from showing some of the savage violence of what was in effect an act of terrorism. Though Lincoln's actual assassin, John Wilkes Booth, was killed after the murder, eight of his alleged co-conspirators were arrested and tried. Their number included Mary Surratt (Robin Wright), a Southern lady who ran a boarding house that had been a meeting place for Booth and his gang.

The war was not yet over and the conspirators were tried by a military court, itself "an atrocity" according to Senator Reverdy Johnson (Tom Wilkinson), one of the film's "good guys". The accused had fewer protections than in a civilian court, and the trial was far from fair.

James McAvoy plays Frederick Aiken, a war hero who takes on Surratt's defence and becomes outraged by the government's willingness to railroad her.

As with many courtroom dramas, at times The Conspirator feels more like a play than a film. Both McAvoy and Penn give strong, naturalistic performances, but the supporting cast have a stiffness common to amateur dramatics.

More serious is the way that Solomon distorts events to make the political message more effective. Then there is the astonishing moment when Surratt tells Aiken that "we're the same", as if there were no moral difference between those who fought to end slavery and those, like Booth, who were willing to kill lest "n*****s become citizens". If the truth is important in wartime trials, then it is also important in depictions of history that purport to teach us high-minded lessons.