Rationalization is God’s gift to mankind. It would be easier to get through the day deprived of oxygen than without the ability to offer self-justifying versions of our actions and character.

In “The Unknown Known,” Donald Rumsfeld, the only man to hold the position of U.S. secretary of defense twice, emerges as a ninja master of the ego-flattering rationale. The film, by master documentarian Errol Morris, asks him to review his career with special emphasis on the Iraq war, and Rumsfeld’s performance is nothing less than dazzling.

A Washington, D.C., power player renowned for his Machiavellian cunning, Rumsfeld served in the administrations of presidents Nixon, Ford, Reagan and George W. Bush. He faced news conferences during the Iraq effort with a grinning confidence that recalled Bugs Bunny yet again outsmarting Elmer Fudd.

His verbal dexterity in the area of de-clarification was unparalleled. His conundrum about known knowns, known unknowns and unknown unknowns is still quoted today.

Gone from public consciousness is the question that triggered that discharge of rhetorical squid ink. At a press briefing five months after the 9/11 terrorist attack, NBC Pentagon correspondent Jim Miklaszewski asked Rumsfeld: “Is there any evidence to indicate that Iraq has attempted to or is willing to supply terrorists with weapons of mass destruction?” The question was evaded. In making the case for war, evidence was irrelevant.

Morris’ film is a fascinating study of a certain managerial mind-set and a wonderful companion to “The Fog of War,” which put similar questions to Vietnam-era Defense Secretary Robert McNamara. While McNamara retrospectively judged some of his own wartime actions indefensible, Rumsfeld is immune to second-guessing.

Morris is admirably evenhanded, never demonizing his subject, but giving him enough rope to hang himself. Rumsfeld, cool and bemused, refuses to knot the noose.

What lessons, Morris inquires, did Rumsfeld draw from his tenure as President Ford’s defense secretary during the end of the Vietnam War? “Some things work out. Some things don’t. That didn’t.” Should the United States have invaded Iraq after the unrelated 9/11 attacks? “Well,” Rumsfeld says, “I guess time will tell.”

Following the Abu Ghraib torture scandal, a series of retired generals denounced him and called for his resignation in newspaper op-ed pieces. Rumsfeld left the Bush administration in 2006, never second-guessing his decisions in office. Smugness? Self-confidence? You decide.

The film is no dry treatise, but a rich audiovisual experience. Tim Burton’s longtime composer Danny Elfman provides an emotion-tugging choral score. Gorgeous seascapes, implying the unfathomable depths of Rumsfeld’s worldview, are a recurring motif.

There are repeated shots of a Washington Monument snow globe whose floating glitter echoes the decades-long blizzard of Rumsfeld’s official memos. Morris asks his subject to explain comments in that paper trail — “snowflakes,” Rumsfeld called them, scores of daily short missives outlining his thoughts on domestic and international policy and bureaucratic practice. Charming as they are, the paperweight’s confetti create zero visibility, obscuring everything in a shimmering blizzard. A crystal ball gone opaque is an apt image for this inquiry into the mind of a man resolutely resistant to being analyzed.