In 2003's Fog of War, Morris focused on Robert McNamara, highlighting McNamara's role as secretary of defense during the Vietnam War.
The Unknown Known -- which shines Morris's spotlight on Donald Rumsfeld -- might well have been called Defense Secretary 2, an attempt to peer into the mind of another powerful man.
Rumsfeld, of course, presided over the U.S. defense establishment during president George W. Bush's invasion of Iraq and the toppling of Saddam Hussein.
A mistake? A waste of lives?
Only time will tell, says Rumsfeld, whose ambitious career often intertwined with that of Dick Cheney.
Rumsfeld is quite different than McNamara, who acknowledged that Vietnam was a mistake, even though his admission arguably qualified as too little, too late.
Less removed in time from Iraq than McNamara was from Vietnam, Rumsfeld remains supremely confident about his decisions. He's never riled by Morris's attempts to catch him in contradictions or lies.
Some of The Unknown Known is devoted to the estimated 20,000 or so memos Rumsfeld wrote while in office. These memos -- called "snowflakes" -- reflect the way Rumsfeld parsed various issues, fretting over "definitions" and "terminology."
Morris persuaded Rumsfeld to read passages from these memos aloud. They'll either strike you as the work of a thoughtful official or a strangely revealing exercise in obfuscation, a man fiddling with language while Iraq burned.
It's pretty clear that Morris remains unimpressed by Rumsfeld's explanations and musings, but Rumsfeld manages to sidestep the intent behind most of Morris's questions. It's also clear that Morris and Rumsfel occupy two different worlds.
Morris, of course, knows how to give a "talking heads" movie pulse. Here, he uses news footage, graphics and a Danny Elfman score as punctuation, a way of keeping the movie from being duller than one of Rumsfeld's memos.
What you think of The Unknown Known may depend on how much time you want to spend trying to figure Rumsfeld out.
He certainly can be cagey: Rumsfeld conveys the impression that Morris (and perhaps all of his critics) have little understanding of the realities he confronted as secretary of defense. He seems to see himself as a well-meaning and thoughtful man whose actions were geared toward accomplishing worthwhile ends in a difficult world.
So why did Rumsfeld want to be in an Errol Morris film anyway?
Rumsfeld tells Morris he has no idea why he agreed to participate in a film with a title based on a Rumsfeldian analysis of what it's possible to know in any given situation and where that knowledge stops.
Maybe Rumsfeld just wanted to prove that he's immune to criticism and that he can't be shaken.
What emerges is a portrait of the ultimate insider, a man of reasonable bearing, who -- many would argue -- presided over unreasonable policies, some of them based on false information.
I'm guessing that most of the audience for The Unknown Known will not be composed of Rumsfeld supporters. I'm also guessing that Rumsfeld probably couldn't care less.