Former defense secretary Donald Rumsfeld goes largely unchallenged in “The Unknown Known,” and could frustrate anyone hoping for answers. (RADiUS-TWC)

The Unknown Known,” Errol Morris’s engaging, purposefully confounding portrait of former defense secretary Donald Rumsfeld, takes its title from one of Rumsfeld’s gnomic, angels-on-a-pin disquisitions that helped make his news conferences during the Iraq War must-see TV for Washingtonians and policy wonks everywhere.

That signature Rumsfeldian pugnacity — and improbable charisma — are still much in evidence in this alternately enlightening and infuriating documentary, which in its structure and subject matter invites immediate comparisons to Morris’s brilliant 2003 film, “The Fog of War,” about Robert McNamara. But as “The Unknown Known” makes clear, Rumsfeld is no McNamara: Seemingly unable to engage in self-reflection, let alone self-criticism, Rumsfeld is given virtually full rein to control the narrative by Morris, who is far more interested in letting the audience dwell inside his subject’s strangely attenuated moral imagination, rather than challenge it.

That’s not to say that “The Unknown Known” doesn’t possess value, as both an artistic exercise in Morris’s oblique, un-prosecutorial brand of filmmaking and as the political history of an era that spans the Nixon administration and the Bush Doctrine. The film traces Rumsfeld’s beginnings as a Navy veteran and congressman in the 1960s, a career during which he became a superbly skilled Washington navigator: a canny, smoothly effective careerist whose love of words (he’s an OED fan) would become crucial to advancing his agenda. At the Pentagon, he treated colleagues to a blizzard of white-paper memos that he called “snowflakes,” in which he asked questions both sharply literal and fuzzily rhetorical, often conducting high-flown inquiries into the meaning of such terms as “guerrilla warfare” and usually ending with a perfectly passive-aggressive “Thanks.”

Through skillful editing and a stirring score by Danny Elfman, “The Unknown Known” invests Rumsfeld’s otherwise banal Washington trajectory with unlikely tension and suspense. What’s more, his maddening habit of pseudo-philosophical speculation fits neatly into Morris’s own ruminative, erudite rhetoric. But if viewers come to “The Unknown Known” hoping for catharsis — or even just a few answers — about Rumsfeld’s role in planning and executing the invasion of Iraq in 2003, they may find themselves leaving more frustrated than rewarded. Faced with the enormity of losses in Iraq and whether the decision to invade was the right one, Rumsfeld has only this to say: “Time will tell.”

Morris doesn’t always let such verbal shrugs be the last word: At one point Rumsfeld flatly declares that the United States doesn’t assassinate people, to which Morris quickly replies by reminding him of the Dora Farms strike, when the military tried to kill Saddam Hussein with four “bunker buster” bombs. But just as often, Morris is inclined to let Rumsfeld have his say unchallenged, without even a follow-up question. Tracing his subject’s tenure as a Middle East envoy in the 1980s, Morris shows the now-famous picture of Rumsfeld smiling and shaking the hand of then-enemy-of-our-enemy Hussein — and there it hangs for posterity, un-remarked or commented upon.

One of the film’s visual conceits is a vast blue ocean that, coupled with huge stacks of papers and card catalogues, suggests the vast written record that Rumsfeld has created, perhaps in part to obscure some vital, hidden — and damning — truth. It’s an elegant metaphor, but rather than ferret it out, “The Unknown Known” leaves viewers feeling just as at sea as when the film started. We might have journeyed inside Rumsfeld’s mindset for a while, but are we richer for the excursion?

Speaking of the Sept. 11, 2001, attacks as a “failure of imagination,” Rumsfeld compares the episode to the bombing at Pearl Harbor, noting that the U.S. military at the time was “chasing the wrong rabbit.” Morris is clearly chasing the right one; in this case, though, the rabbit was clever enough to get away.

★ ★ ½ PG-13. At Angelika Film Center Mosaic and Avalon. Contains some disturbing material and brief nudity. 103 minutes.