War is hell. More than that, war is evil-ask almost any soldier who's been there. Not many of them come through combat with anything other than loathing for organised violence. But not many politicians or defence department bureaucrats ever ask ordinary soldiers before ordering them into battle, or taking steps that will lead to war.

They wouldn't get the right answers. Last year's documentary, The Fog of War might just be a substitute for those battlefield opinions. I wonder if anyone in the Royal Nepali Army or the government has seen it. The film is based around a series of interviews with Robert S McNamara who was US secretary of defence for seven years in the 1960s. He first worked for President John F Kennedy then stayed on in his post after Kennedy was assassinated in 1963. His was a lively watch. First the Cuban missile crisis brought the world to the brink of nuclear war. McNamara and Kennedy agonised over the presence of Soviet nuclear missiles in Cuba, choosing to confront Moscow with a fierce stance in public, while offering private concessions. It worked and we were spared both a devastating war and the presence of atomic weapons 140km from America.

Then came Vietnam. About 16,000 American troops were already there when McNamara assumed office. In The Fog of War, he relates how that number climbed steadily as he bought into the theory that the communists in North Vietnam were somehow a threat to the US, that their victory in their own country might prompt other nations to choose communism over liberal democracy.

McNamara has many regrets about his days of war mongering. He speaks frankly about them in the film, but says in the end, war has its own dynamic. He explains the title of the film as a reference to the way that truth is obscured and twisted when a nation is at war. The secretary of defence admits to a fair amount of twisting in his time. Coming back from one of his many trips to South East Asia, a younger McNamara tells the media that the war is going in America's favour and the communists are on the run.

We knew then that he was lying and time hasn't diminished the falsehood.

The chemical defoliant, Agent Orange, came into wide use during McNamara's time as secretary of defence. Chilling footage of soldiers rolling drums of the stuff onto aircraft illustrate his mea culpas, his insistence that had he known more about the effects of the poison on people, he might not have approved its use. In 1967, McNamara was fired by his boss, President Lyndon B Johnson. The war wasn't going well. Young Americans were protesting in the streets, sometimes violently. The media was showing people across the US portraits from the theatre of war that chilled and shocked many. McNamara was eased out of high office but nothing changed.

In the end, nearly four million Vietnamese died in the 'American War', as it's known in Vietnam. Tens of thousands of homes and villages were destroyed, vast swathes of countryside laid waste. Countless billions of dollars were spent in what was-ultimately-a failed attempt to stop communism. Because The Fog of War is an American film, McNamara is not asked whether he considers himself a war criminal. The tone of the documentary is studiously neutral, and perhaps this is best.

But war, all war, is evil. That's the message I take from this film. War happens because politics fails, or is made to fail by evil people. There is always an alternative to war.