You have to be intrigued by the prospect of Errol Morris, a documentarian with an undisguised left-wing agenda, sitting down with Iraq War Defense Secretary Donald Rumsfeld. But Morris is likely to disappoint liberals in “The Unknown Known” by failing to take down an apparently weak target.

Morris is heard badgering Rumsfeld on camera about torture, WMD intelligence and the handling of the war. His subject, though, earns at least a draw. He answers everything with a steady gaze and that combination of gosh-darn Midwestern sunniness and sardonic dismissiveness.

It’s hard not to conclude that even Morris (who holds back somewhat on the snark, though not on the corny reenactments and heavy-handed musical cues) stumbled into respecting a man who simply believes what he says and can no more alter his principles than his personality.