It's stretching a point to call The Unknown Known a "sequel" to The Fog of War, but they make for a hell of a double feature. At a sufficient remove, the films are all but identical: Errol Morris, one of the great pop-journalist documentarians of the modern world, interviews a controversial U.S. Secretary of Defense, allowing him to narrate the story of his life and career, and especially giving him room to explore the ramifications of the notorious war that flowered under his care; all driven by a repetitive, propulsive score (Philip Glass then, Danny Elfman now, with a sort of Elfman/Glass hybrid that's by light-years the most interesting thing he's composed in at least a decade). And if that was that, it would still make for a remarkable and greatly useful project: honestly, if Morris wanted to flesh out his "Portraits of the Defense Secretaries" series with all the rest of them, even the ones who didn't help plunge the United States into military quagmires, I'd be there for each and every one of them.

But here's what matters most: The Unknown Known isn't simply The Fog of War with Donald Rumsfeld subbed in for Robert McNamara. Taken as a pair, the films represent two wildly divergent ways of thinking about the past: McNamara is reflective and anxious for absolution, while Rumsfeld is patiently icy, expressing nothing that betrays a hint of curiosity or introspection about his tenure as George W. Bush's Secretary of Defense during the plagued early years of the Iraq War. Like the earlier film, it's a character study and political history rolled into one, but it might honestly be even more fascinating as cinema: for Rumsfeld proves to be a complicated, slippery subject, and the film frequently turns into a form of verbal fencing between him and the unseen but often-heard Morris, with the director constantly trying to find some new angle with which to attack the ex-secretary's one-sided storytelling, only to have Rumsfeld easily turn Morris aside with a rhetorical flourish that turns their conversation into a meditation on what words actually mean, all without ever loosening his toothy, disarming smile even slightly.

The Unknown Known delights in exploring sophistry and fine distinctions of language that Rumsfeld frequently seems to be making up on the spot (in fact, the famed quote that gives the film its title comes under the scrutiny of the subject itself: he claims that "unknown knowns" refers to one thing, at which Morris gently points out that Rumsfeld's original memo claims the exact opposite, at which point Rumsfeld furrows his brow, re-reads his original wording, and airily concludes that the memo was wrong, as idly as one might choose between two barely distinguishably shades of white paint). In the one flashy stylistic gesture that Morris ever makes, he occasionally throws up the dictionary definition of some key word Rumsfeld says onscreen, as Rumsfeld continues to talk, a kinetic and purposefully distracting way of pointing out that a) words have set meanings; b) those set meanings don't matter if you can talk fast enough to outpace them.

All this dancing with language clearly frustrates Morris, who has never been so audible, nor audibly annoyed, in any of his movies; no Fog of War is this, where the subject allows himself to be analysed, nor Mr. Death, where the subject doesn't realise that he is spilling his guts. Rumsfeld is guarded and artificial, refusing to speak one syllable that he hasn't vetted in his head. He does one thing, in fact, that I can't remember ever having seen in a documentary: when Morris holds before moving on, leaving a long conspicuous silence, Rumsfeld doesn't fill it. That's a rudimentary interview trick: let the subject fill an uncomfortable pause with an expansion or clarification or contradiction of what they just said. Morris waits, and Rumsfeld just gazes pleasantly at him - at us, since the film has been shot in the way of all recent Morris documentaries, where the subject appears to be looking the viewer in the face - and refuses to even twitch a muscle in his mouth. And Rumsfeld, one of the great politicians of the modern age at using journalists' tricks against them, clearly knows that's what he's doing: even without his expression changing, you can see his eyes smile.

What we get, therefore, is not a film about the Iraq War and how it was sold, nor a film about what the ramifications of the war have been in the intervening decade. And this has irritated some viewers, expecting any kind of historical accounting - even Morris tries when he can to put an editorial spin on things, most prominently when Rumsfeld is speaking about how "they" - meaning the Iraqi government - refused to take any steps to avoid war, while the director holds on a fish-eye panorama of Washington, D.C., clearly hoping to ironically make us think of the other "they" who didn't try very hard to avoid conflict. But's that simply not what The Unknown Known is about, and Morris does clearly realise this, by the way he structures most of the movie around word choice and language usage. It's a film about how powerful men represent themselves to the world and to history, selecting which facts to acknowledge and defining concepts in whatever way best suits their purposes. Morris is clearly no fan of Rumsfeld or his politics - a fact that informs the film's surprisingly funny last beat - but The Unknown Known doesn't fashion itself as a liberal message movie, nor as specifically anti-Rumsfeld.

Instead, the perspective adopted here seems to be not one of judgment, but observation: this is the way the mind of this man works, and it is why he was able to perform the actions he did that had the enormous international repercussions that we still live with. But that's not the film's focus: it has a timelessness that goes far beyond its failures to trick Donald Rumsfeld into acknowledging remorse. Indeed, the fact that Morris's subject was so perfect at obfuscating and stonewalling is what makes The Unknown Known far more interesting than just a mere contemporary issues documentary. It is about the nature of politics and power and how they are secured by wit and control, and as such it has a sick fascination that's not quite like any other documentary I have ever seen.