Tony Blair hoped war in Iraq would not be necessary and Saddam Hussein would be removed by diplomatic pressure, his former foreign affairs adviser says.
Mr Blair has always strongly defended his policy in Iraq
Sir David Manning said the former prime minister had not committed to the war at George Bush's ranch in 2002.
But he told the New Statesman the way the post-war situation was handled had been a failure.
In March 2003 Mr Blair won Parliament's backing to send British troops to war - despite a rebellion by 139 Labour MPs.
In the interview Sir David, who is soon to step down as British ambassador to Washington, denied Mr Blair had already committed to war nearly a year before.
He said Mr Blair "was always in favour of regime change, but that did not mean he always wanted regime change through military means.
"He must have known it might come to military action, but I have always believed he hoped and probably believed there was a way of getting there by using the UN to put pressure on Saddam.
"I don't think he ever wanted to go by the military route."
He added: "He didn't talk to me as a prime minister saying to me: 'I've made up my mind...we're going to war with Iraq'."
He also said Mr Blair had believed there were weapons of mass destruction in Iraq and had not fabricated concerns to justify the policy of going to war.
Sir David said Mr Blair may have put too much weight on assurances from the US that plans for post-war Iraq had been drawn up by Colin Powell's State Department, when in fact he was sidelined by Donald Rumsfeld.
Within weeks of the fall of Saddam Hussein there were serious concerns about the way the US was handling the occupation, he told the magazine.
"I don't think anybody can see that the immediate post-war situation was anything other than a failure," he said.
"We had hoped that rapidly the situation would stabilise, that it would be possible to introduce reconciliation, get the economy moving quickly and rebuild society.
"Did it happen quickly? No, we failed. We were over-optimistic."
Mr Blair was succeeded by Gordon Brown in June. As prime minister he faced criticism from within his own party over his policy in Iraq, but always strongly defended his interventionist foreign policy.