He told the inquiry there never was a "precipitate rush to war" despite the close ties between Blair and George Bush, the US president at the time.
"You seem to be wanting me to say that Tony Blair signed up to say regardless of the facts, regardless of WMD [weapons of mass destruction], we are just going to get rid of the guy [Saddam].
"It was not like that," he said.
But he added that it was clear throughout that if the diplomatic route didn't work then the military option was evident.
The former aide began his three-hour evidence session by defending his close relationship with Blair.
"If he asked me to jump off a building, I wouldn't have done it, if he asked me to do anything silly or stupid, I wouldn't have done it, but he never did," he said.
|John Chilcot, chairman of the inquiry, has insisted it will not be a whitewash [AFP]
He also said Gordon Brown, the current British prime minister who was finance minister at the time of the Iraq war, would "absolutely" have been one of the "key ministers" Blair discussed decisions on Iraq with.
Blair is due to appear in late January or early February, while Brown will not be called until after this year's general election, which is expected to be held in May.
Campbell, Downing Street's former director of communications and strategy, resigned in August 2003, the month after Dr David Kelly, a ministry of defence weapons expert was found dead near his home with slashed wrists.
Earlier that year, he fiercely denied a BBC report that he "sexed up" a dossier claiming Iraq could launch a chemical or biological attack within 45 minutes to help justify the war.
Campbell was accused of leaking the name of Dr Kelly as the source of the story, which he denies.
An official inquiry - one of several probes to which he has already given evidence over the Iraq war - subsequently exonerated him over the affair.
Blair is expected to give evidence sometime in the two-week period from January 25.
The former prime minister faced a major backlash in Britain over the decision to support former US president George Bush over the war.
Blair insisted last month he would have supported the war, which did not get United Nations approval, even if he had known Iraq did not have weapons of mass destruction.
He resigned in 2007, having led his Labour party to three election wins.
The inquiry, which started hearing testimony in late November, has faced criticism that it is not questioning witnesses rigorously enough.
Critics of the invasion had long demanded an investigation into whether the war, which was extremely unpopular in Britain, was illegal.
Many were disappointed when it was announced that the inquiry had no power to apportion blame or establish criminal or civil liability.