As the U.S. mulls intervening in the Middle East again, former defense secretary Donald Rumsfeld wriggles out of straight answers on the Iraq war in Errol Morris’s new documentary, screening in Venice Wednesday.

“The Unknown Known” takes its title from a 2002 speech Rumsfeld gave to justify the invasion.

Asked at the time whether Iraqi leader Saddam Hussein possessed weapons of mass destruction, he replied that there are “things we do not know we don’t know.”

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In the film, Morris repeatedly attempts to quiz Rumsfeld on this and other decisions taken by a man accused of disastrous mismanagement of the war and condoning policies on interrogation which led to cases of extreme abuse of prisoners.

His interviewee proves to be simultaneously combative and a slippery character.

The film is structured around Rumsfeld’s obsessive memo writing, with the 81-year-old reading several of his so-called “snowflakes” aloud.

Unlike the director’s 2003 “The Fog of War” documentary about the career of Robert S. McNamara, secretary of defense during the Vietnam War, Morris’s subject here offers no confession but a series of chilling self-contradictions.

It is a portrait of a man caught lying through his teeth, who smiles it off.

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In a one-to-one interview based on 33 hours of footage, Morris asks him whether interrogation tactics signed off by the defence secretary led to abuse, and if the Bush administration linked Hussein to the 9/11 attacks.

Rumsfeld denies both — though Morris has an independent report on the abuse scandal at Abu Ghraib prison which proves otherwise, and a tape of Rumsfeld at a press conference clearly saying Hussein was involved in the Twin Towers attack.

“Look, absence of evidence is not evidence of absence. Thinking that is a lack of imagination,” Rumsfeld says.

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After nearly two hours, the documentary — which also charts Rumsfeld’s rise and uncanny ability to manipulate himself into positions of power — ends with the octogenarian shedding an unconvincing tear for a wounded soldier he visited.

“My wife describes Rumsfeld as the Cheshire Cat from ‘Alice in Wonderland’, who at the very end vanishes and is left with just a smile. I think it’s an apt comparison,” Morris told journalists in Venice.

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“I look at it as a devastating portrait, a frightening portrait,” he said.

“I contradict him quite often. But that’s not the goal, I much prefer it when he contradicts himself, which he does repeatedly.

“He uses a strange language designed to manipulate others, but ends up losing himself in a sea of words,” he said.

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Morris’s final question in the documentary, one that the viewers were also asking, is why Rumsfeld agreed to be interviewed in this fashion in the first place.

True to nature, the white-haired veteran politician avoids answering.

“I read about the ‘snowflakes’ he had written on 40 to 50 years in politics and business and decided there was a movie there,” Morris said.

“I wrote him a letter. His lawyer said I was mad, that Rumsfeld would never do it. Yet I got a reply within a week to visit him in Washington,” he said.

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“I didn’t get a confession. Do I think I got the real Rumsfeld? I think one of the great mysteries of the film is whether he is acting or not,” he added.

Watch a preview for “The Unknown Known,” posted by VICE Magazine, below.