Mr Howard said the belief Saddam Hussein had possessed WMD was ''near universal'' at the time. He also argued that opponents of the war might now be claiming vindication in the light of continuing chaos in Iraq, but that to have left Saddam in place would have seemed ''oddly passive'' to Americans who in 2003 were still coping with the ''atmosphere of vulnerability'' that took hold after the attacks on the US on September 11, 2001.

'Necessitated a 100 per cent ally': John Howard said Australia was obliged to follow the US into the 2003 Iraq war. Credit:Marco Del Grande

Mr Howard acknowledged mistakes were made by the US-backed Coalition Provisional Authority which ruled Iraq immediately after the war, citing in particular ''problematic'' decisions to disband the Iraqi army and cleanse the country of the Baathist party.

He said the bloody conflict between Sunni and Shiites which broke out in Iraq after the war ''did more damage in my judgment to the credibility of the coalition operation … than the failure to find stockpiles of WMD''. He also acknowledged his decision to enter the war had polarised opinion.

But the circumstances of the time, he said, ''necessitated a 100 per cent ally, not a 70 or 80 per cent one''.