It is a great shame that the excellent documentary dissection of the complicated and rather dangerous mind of former US Secretary of Defense Donald Rumsfeld, The Unknown Known, has had such a limited release across the US this month. It undoubtedly has more to say about the nation's recent history and the risks of its future decision-making than a thousand Hollywood blockbusters.

Many Americans likely feel the need to go on blissfully forgetting a time when, as a soundbite in the film reveals, Rumsfeld had a staggering 80% public approval rating and, it is painful to now recall, was frequently discussed as being something of a sex symbol.

The Unknown Known, which is scheduled for an Australian debut at the Sydney Film Festival in June, has nonetheless caused a stir among foreign policy watchers and otherwise politically aware types for whom the Iraq War and other events of the Bush years remain as much a warning as troublesome memory.

It works particularly well as a companion piece to director Errol Morris' earlier documentary The Fog of War; another long-form interview with a multi-administration Secretary of Defense, in this case Robert McNamara, who served under Kennedy and Johnson.

Where that film showed how even wholly rational actors could make poor decisions as a result of the eponymous opacity of geopolitical conflict ('it isn't that we aren't rational, we are rational, but reason has limits' as McNamara puts it'), The Unknown Known is more an exploration of how irrational actors like Rumsfeld can create their own disastrous fog or, in this case, a blizzard of blinding and largely blind thoughts and half policies relayed through millions of 'snowflakes', as Rumsfeld called his constant memos to those around him.

It is therefore a stark reminder of the importance of personality and character in politics. When Gerald Ford installed Rumsfeld as Secretary of Defense in 1975, Rumsfeld began dismantling the structures of Soviet détente built up by Richard Nixon and Henry Kissinger in favour of a more muscular 'peace through strength' that would define his actions over the next 30 years.

As the film reminds us, Rumsfeld served in many of the world's crucibles during those three decades, including as special envoy to the Middle East in the 1980s. During this time, we learn that he advised President Regan that the US 'ought to lighten our hand in the Middle East' and 'move away from the current situation where everybody is telling us everything is our fault and angry with us, to a basis where they are seeking our help.'

While viewers might expect a re-reading of this memo to present a moment of pause, considering his later enthusiasm for ousting Saddam Hussein's Iraqi regime by any means necessary, Rumsfeld shows himself to be possessed of a rare self-belief that can even accommodate the claim that he applied a 'measured, nuanced approach' to the eventual invasion.

Despite his vast experience, and the emphasis he places on the proper gathering of intelligence as a means of avoiding future US disasters such as Pearl Harbor and September 11, Rumsfeld comes across most clearly as a weak analyst of global affairs and of the ability of US strength alone to impose order and stability.

Many viewers will bristle at his casual dismissal of some of the worst excesses of the War on Terror, including the flimsy premise for the devastating Iraq invasion and the circumvention of the Geneva Convention in refusing prisoner of war status to detainees.

They might conversely enjoy seeing Rumsfeld presented with firm evidence of his great mendacity, including the fact that the Bush Administration essentially directed more than two-thirds of Americans into believing there was a straight line between Saddam Hussein and Osama bin Laden.

It could even prompt laughter when Rumsfeld gets lost in the infamous word game of knowns and unknowns from which the documentary takes its title, or ties himself in knots discussing the attempted assassination of Saddam at the commencement of the 2003 invasion. This, he tells us, was not an assassination attempt, which would have contravened US policy, but an 'act of war' that could have 'prevented war'.

It would be fun, that is, if the implications weren't so serious. Because they are, it is better to see The Unknown Known as a reminder of the importance of letting cool heads prevail in military decision-making. Returning to The Fog of War, it is frightening to think how the Cuban missile crisis might have panned out had it been Rumsfeld in the room with Kennedy and not the more rational McNamara.

In that film, McNamara showed himself willing to admit to his mistakes. Nearing the end of his life (McNamara passed away six years after the documentary's 2003 release), he was also able to impart wisdom on the nature of conflict and its possible avoidance, including, most famously, the need to 'empathise with your enemy'.

The Unknown Known leaves it firmly up to the viewer to decipher lessons, with Rumsfeld's snowflakes representing no more than instant responses to events, informed by ideology rather than any sense of analysis or consensus gathering. Asked by Morris 'wouldn't it have been better not to go there (Iraq) at all?' he responds that 'time will tell', seemingly oblivious to the great weight of public opinion that had already come to a firm conclusion on the matter. Twice throughout the film, Rumsfeld also calls out what he sees as the 'irrational' decision-making of the Iraqi regime in response to the threat of war. Be forewarned – the lack of self-awareness can be staggering.