The Unknown Known is a fascinating and frustrating look at the political life of one of the more controversial figures to haunt the halls of the White House.

Make a list of the most divisive politicians during the 21st century, and Donald Rumsfeld will feature near the top. Although he first began his political career as a congressman in the early 1960s, it was his tenure as Secretary of Defence for the recent Bush administration that brought him to the attention of many.

Of course it was during that tenure that the decision was made to go to war in Afghanistan and Iraq. While Afghanistan was an easy sell (with that country the location of the Al Qaeda terrorist group directly responsible for the 9/11 attacks), the Iraq invasion was not, with the issue of increasingly sceptical intelligence on “weapons of mass destruction” (which was the lynchpin for the war) a weight that sank a government’s credibility.

It was during a moment where Rumsfeld defended the Iraq war in front of a hostile media pack that defined many feelings towards the former Secretary of Defence, with his now infamous “There are known knowns…” quote perplexing many and inspiring the title for this documentary, which is directed by ace documentarian Errol Morris who brings some of his own “known knowns” to the proceedings.

Rumsfeld’s relationship with words is of major importance here, with his hundreds of thousands of memos written during his political life (all now boxed away in a stunning image of paperwork gone epic) used as fuel in this part investigation, part inquisition and all around fascinating experiment into trying to dig underneath a deeply buried stone wall of charismatic defiance.

Morris is clearly not a fan. His 2006 documentary Standard Operating Procedure delved into the controversial interrogation techniques used on terror suspects under Rumsfeld’s watch, and his questions come direct and are backed with evidence.

Rumsfeld is game, with that straight forward nature often catching Morris during biased moments. “Who is this ‘they’?” Rumsfeld asks when quizzed about the American government’s decision to invade. Morris dutifully rephrases it to “why did we invade?” Score one for the guy in the hot seat.

Yet it doesn’t often go that way. Morris is a king of his craft, of which is to let the camera soak in all of Rumsfeld’s being – his face, his voice, his reasoning – that leaves us with a much more charismatic impression of the man.

But it also reveals a man that has been entirely transformed into a political machine who says a lot yet reveals nothing, and a man obsessed with the notion of “failure of imagination” (as in there could be WMDs) yet doesn’t leave much of an impression, save for a bitter after taste and the memory of a smile that’s been though too many fundraisers and grilling’s from the press.

When Morris asks why he even agreed to do this documentary, Rumsfeld answers “I don’t know.” Only fitting that the most honest answer is saved for last.