Joan Smith: In 'Loyalty', truth and fiction are one and the same

I've never really believed that Tony Blair lied about why he wanted to go to war in Iraq. I've always thought he convinced himself that Saddam Hussein had WMD – not that difficult for someone who believes in an invisible deity – and dismissed people who thought differently. Now my instinct has been confirmed by Sarah Helm's play Loyalty, which I've just seen at the Hampstead Theatre in London. At the time of the war, Helm lived with (and is now married to) Jonathan Powell, Blair's chief of staff. It was Powell's job to monitor telephone conversations between Blair and important people, and in the play, "Laura", the character loosely based on Helm, listens in.

Here, in the play, is Bush talking to Blair, less than a month before the invasion: "But you know the Germans have got some really good new stuff showing he has those biological weapons. You've seen that, right?" Blair responds hesitantly that he's just heard about the new source. Bush insists that "the German stuff shows that son of a bitch is really ready to offload. No doubts now, Tony." Bush flatters Blair, praising his body language and deriding opponents of the war. "I mean, do these people not care about the citizens of Iraq? The answer is no. But when that son of a bitch hits Europe, they'll be saying: 'Where were George and Tony?'"

In the play and in real life, the German source was an Iraqi defector who later admitted that he lied to encourage the overthrow of Saddam. When I saw the play, one of the biggest audience reactions came when Blair takes a call from Rupert Murdoch, passing on a message from Donald Rumsfeld about needing to use the British base on Diego Garcia. The scene was written a couple of years ago.

Sarah Helm is a friend of mine and a former Independent journalist. (There's a nice moment in the play when the head of MI6 remarks that Laura writes for The Independent, and makes the sign of the cross; it reminded me that Tony Blair once glared at me at a No 10 party when I reminded him that I wrote for the paper.) The play is described as a "fictionalised memoir" and it has clearly puzzled critics, who seem to have missed the extraordinary insights it offers into Bush and Blair's relationship; they don't seem to realise either that the painful conflict between Laura and Nick (the Powell character) was played out between other political couples who found themselves on opposite sides of the argument. My partner at the time was a government minister, and I recall Blair's clique forever insisting "everyone knew" Saddam had WMD.

It wasn't true, but pro-war politicians existed in a self-reinforcing circle. The idea that anyone who opposed the war must support Saddam came up repeatedly, and is perfectly illustrated by Nick's preposterous question to Laura: "So you want your friend Saddam to stay in power, do you?" The strain it placed on people who cared deeply about each other may go some way to explain why Sarah decided to write Loyalty.

She's been accused of the opposite, but I think that is unfair. She's been true to her passionate conviction that the war was wrong and to her belief in her husband, despite disagreeing with him on one of the most controversial foreign policy decisions of our lives. Her drama says more about the psychological processes enabling the conflict than any number of factual accounts of how it started.;