As politicians lapped up the credit and thousands of people across America took to the streets to revel in the perceived victory, the ones who were truly responsible for the capture and killing of Osama Bin Laden remained behind the scenes.

Their moment has come in director Kathryn Bigelow and screenwriter Mark Boal’s complex, challenging and totally gripping Zero Dark Thirty, the follow up to their Oscar-winning Iraq war drama The Hurt Locker.

Those expecting or hoping for a flag-waving, ultra-patriotic exercise in easy-to-swallow triumphalism will need to look elsewhere. What we get is a sobering, cerebral and, most importantly, apolitical account of the tireless work that went into tracking down America’s public enemy number one by a small band of Central Intelligence Agency and military personnel.

Zero Dark Thirty is a classic procedural in the mould of All The President’s Men and TV’s The Wire, with the crime here being the 9/11 attacks. Bigelow’s first masterstroke is not to show us those infamous images (they’ve been scorched into our psyches already); instead the film opens with a chilling montage of overlapping telephone calls made by people trapped in the Twin Towers played over a black screen.

The film then jarringly throws us into an undisclosed CIA site in Pakistan where new girl Maya (Jessica Chastain) witnesses ‘enhanced interrogation techniques’ (read: torture)  up-close for the first time courtesy of fellow operative Dan (Jason Clarke), a chameleon who looks as comforable in a shirt and tie as he does with his hands around someone’s neck.

Dan wants actionable intelligence and makes good on a repeated threat to his prisoner that “when you lie to me, I hurt you” by subjecting him to waterboarding and other forms of torture. Initially distressed, Maya is soon employing similiar tactics in her own interrogations.

Maya is a blank canvas, with no back story, personal life or friends, merely an all-consuming, evangelical zeal to find Bin Laden by any means necessary. She represents a post-9/11 America whose moral compass has been eroded by a willingness to justify increasingly unethical behaviour.

The only connections she has are with fellow CIA officers, in particular Jessica (the excellent Jennifer Ehle), an older, more wily operative who talks of baking a cake for a potential informant and having him killed if he doesn’t prove useful in the same conversation.

As coldly analytical as Maya and her colleagues are towards contacts or detainees, seeing them as nothing more than assets to drive forward the investigation, its constant dead-ends, labyrinthine complexity and mounting casualties breeds a frustrated thirst for vengeance. At her lowest ebb, a grieving Maya coldly informs a colleague: “I’m going to smoke everybody involved in this op, and then I’m going to kill Bin Laden.”

In the hands of a lesser director, Zero Dark Thirty could have been reduced to a tub-thumping embarrasment, but Bigelow is too smart to poison Boal’s painstakingly researched script like that and instead maintains a measured detachment to the material.

The Navy Seal raid on the compound believed to house Bin Laden (only Maya is prepared to stick her neck out to categorically state he is in there) is a case in point. It could so easily have turned into a hyper-stylised action set piece akin to a video game, but in Bigelow’s hands Zero Dark Thirty‘s final act has a documentary feel, all be it a superbly filmed one (much of it shot using night vision lenses) that has a taut, nerve-jangling authenticity to it.

Bigelow is clearly fascinated with the dehumanising effect the so-called ‘War on Terror’ has on those fighting on the frontlines. In The Hurt Locker it was a bomb disposal expert; here it’s Maya whose soul is gradually eaten away by the things she sees and does.

During one scene Maya, angry at a colleague’s decision not to deploy a team to track down a key target, realises her bad cop approach isn’t working and gives him a beer to sweeten him up. It’s reminiscent of an earlier moment when she observes Dan using the same tactic to get a tortured detainee onside and serves as a subtle allusion to the lengths she’s prepared to go.

The fact that politicians and pundits on all sides have come out against the film means Bigelow and Boal have done their job. In one scene, Maya and Jessica watch impassively as President Obama states on TV that “America doesn’t torture” before carrying on their conversation. Just as with the rest of the film, Bigelow leaves it to us to read into it what we will.

The most explosive charge levelled at the filmmakers has come from those attacking Zero Dark Thirty for supposedly condoning torture (in many cases before they’ve even watched the film). Hard to watch they may be, but to omit these scenes from a film as important as this because they make politicians uncomfortable would have been to disregard a key chapter and inexorably damage the story Bigelow and Boal are trying to tell.

A fantastic cast is led by the compulsive Chastain, who knows exactly what to give in every scene. It’s a haunting performance, one appropriate to a film that, like Robert Redford at the end of The Candidate asks “what do we do now?”.