Author John Kampfner believes "people are being a little bit too simplistic about Chilcot", which is published today.
"I think we're going to learn a lot that's new, that's the detail, the big picture we pretty much know," Kampfner told Julia Hartley-Brewer.
"We know Tony Blair had locked himself on to George Bush's radar the year before he was elected. Everything else that happened after that was Blair saying to the American's 'I'm with you and trying to get the Brits to go along', and he did whatever it took."
Many expect Tony Blair to be heavily criticsed.
"All this stuff about did Blair lie or not, they need to get on to a psychiatrists couch about when is a lie a lie, what happened was that Blair believed this stuff, and don't forget also pretty much the whole British public were up for it, the Tory opposition was baying for Saddam.
"People are being a little bit too simplistic about Chilcot, it's basically will he nail Tony Blair as a war criminal and a liar? And if he doesn't will it be a whitewash? Well actually the truth is it's a bit more complicated than that."
The author of Blair's War, believes inappropriate practices meant the war wasn't properly planned.
"What happened is they all got swept along. The processes became absurdly informal so for the government, all these people in a rather pathetic way trying to ingratiate themselves with the Prime Minister instead of doing what they should have done and given him objective cold advice.
"What was absolutely not permittable was the shortcuts and the sloppy practices that allowed Tony Blair to will the country to war, rather than doing it in a hard-headed way.
"One of the absolute basics was you have to know what success looks like, you have to have a exit strategy and you have to have a strategy for when you're in there. He went to war on a wing and a prayer."