He added: "Tony Blair is always and ever an advocate. He makes the most persuasive case he can. Not departing from the truth but persuasion is everything. Advocacy for my position, 'my Blair position'."
He said Blair gave the case for war based on his own assessment of the circumstances, "not on the fact, what the assessed intelligence said".
Chilcot was also critical of Blair's readiness to follow then US President George W. Bush's lead.
"Tony Blair made much of, at various points, the need to exert influence on American policy making," he said.
"To do that he said in terms at one point, 'I have to accept their strategic objective, regime change, in order to exert influence'. For what purpose? To get them to alter their policy? Of course not. So in effect it was a passive strategy. Just go along."
Commenting on the revelation by the inquiry that Blair wrote to Bush in 2002, "I shall be with you whatever", Chilcot said his reaction on first reading the note was: "You mustn't say that!"
He added: "Because you're giving away far too much. You're making a binding commitment by one sovereign government to another which you can't fulfil. You're not in a position to fulfil it. I mean he didn't even know the legal position at that point.
The inquiry concluded that Blair overstated the threat posed by Iraq leader Saddam Hussein and the invasion was not the "last resort" action presented to Parliament, when it backed the action, and the public.