Leading article: Chilcot is now set for one crucial piece of testimony

A leopard cannot change its spots. And, true to his nature, Alastair Campbell decided yesterday that the best form of defence was attack. In his evidence to the Chilcot Inquiry on the invasion of Iraq, the former Downing Street communications chief flatly contradicted evidence given by some of the senior civil servants called previously.

The former Washington ambassador, Sir Christopher Meyer, had been wrong, according to Mr Campbell, to claim that Tony Blair made a key decision to support regime change in Iraq at a meeting at President George Bush's Crawford ranch in April 2002. Mr Campbell rebutted the claim made by Sir John Scarlett to the inquiry that the intelligence chief had lacked the authority to challenge the Prime Minister's misleading forward written for the notorious September 2002 dossier which laid out the threat posed by Saddam Hussein's weapons of mass destruction.

And Mr Campbell strenuously rejected allegations that that he himself had interfered to "beef up" that dossier. Looking back on the whole business, Mr Campbell's verdict was: "I was privileged to be there and I'm very, very proud of the part that I was able to play".

Much as this unapologetic performance will enrage opponents of the invasion, none of it should be remotely surprising. Mr Campbell was never likely to throw up his hands and admit that Mr Blair was set on regime change in Iraq regardless of international law, or that he knowingly and deliberately exaggerated the intelligence warnings about Iraq's weapons.

But what did we learn from Mr Campbell's testimony that we did not know before? The answer to that is: not a huge amount. We gleaned a little more about Mr Blair's thinking and behaviour in the build up to the invasion. According to Mr Campbell, the Prime Minister sent letters to President Bush about the Iraq disarmament strategy, the gist of which was: "If that cannot be done diplomatically and it is to be done militarily, Britain will be there".

But despite some forensic probing from the Chilcot panel than has not hitherto been in evidence, Mr Campbell yielded virtually no new information. And it is becoming increasingly clear that there is only one testimony that will truly matter in this process. Influential though he might have been, Mr Campbell was ultimately a media spokesman. He was making the case, not the decisions on Iraq. The true significance of Mr Campbell's testimony – as with all the the testimony Chilcot has thus collected thus far – is the questions it raises for Mr Blair when he is eventually called to give evidence to the inquiry.

The former prime minister will need to justify the style of his government, which has become apparent through the Chilcot inquiry, from the informal decision-making, to the secret memos, to the only sporadic consultation with cabinet ministers. He will need to answer hard questions about the presentation of the case for war, in particular why he said in the September 2002 dossier that the intelligence on Iraq's weapons was "beyond doubt". He will need to explain what planning he demanded be put in place for the aftermath of the invasion. And, most important of all, he will need to explain the timing of his fateful decision to support military action by the US against Iraq.

Combative and bruising as yesterday's exchanges were, the main event of this inquiry is still very much to come.