Leading article: A proper purpose for Chilcot

The Iraq inquiry hearing on Tuesday will be a trial for Sir John Chilcot and his colleagues as much as for the star witness. If Sir John is serious about his pledge to demand more rigorous questioning during the second phase of his inquiry, he could hardly ask for a better test than Alastair Campbell. Four inquiries have scrutinised the former Downing Street spin-doctor's involvement in making the case for war. None has managed to lay a glove on him, yet Mr Campbell's role remains shrouded in suspicion.

No doubt Mr Campbell will rehearse the detailed account he gave to the Hutton inquiry of his role in compiling the dossier on Iraq's weapons of mass destruction. But there were weaknesses in his rebuttal of claims that he was at the heart of the operation which need to be subjected to further rigorous examination. How much of the dossier was produced by amateurs, including Mr Campbell and his team of political advisers, rather than the intelligence experts? What pressure did they put on the experts to make the mistake of including the claim that Saddam could deploy unspecified weapons within 45 minutes? Ultimately, the charge is that a document intended to give a dispassionate analysis of the threat presented by Saddam was transformed into a political manifesto for war, during a series of drafting meetings attended by government spin-doctors.

Mr Campbell is hardly likely to break down and admit that he did anything wrong. Any more than is Tony Blair, expected to give evidence in the next few weeks. The arguments about the case for war have been fought to a standstill, and are well known on both sides – with, we would suggest, most people accepting the case made by this newspaper before it began, that it was illegal, immoral and indefensible.

In fact, it is away from the star witnesses that the Chilcot inquiry is conducting its most valuable work. Any lessons to be learnt from the way Mr Blair sold his war have been learnt long ago: that Britain should hesitate long and hard, and exhaust all alternatives, before engaging in military action abroad in the absence of an immediate and grave threat.

However, as this implies, The Independent on Sunday is not opposed to military action in all circumstances. There was a purpose to Labour's "ethical foreign policy" before Iraq that was admirable and right. Robin Cook was a liberal interventionist in Sierra Leone and Kosovo. And in Afghanistan, our military involvement was initially justified, when the aim was to replace the Taliban and bring the al-Qa'ida leadership to justice. This newspaper became the first last year to call for British troops to come home because their mission has become diffuse and unclear. Indeed, our report two months ago that British troops are planning to withdraw to the centre of Helmand and let the US take over appears to have been confirmed last week. This suggests that the military authorities accept our argument.

In some cases, which should be defined more carefully than they were in Iraq and with a better exit strategy than in Afghanistan, intervention is justified. That is why it is so important to learn the lessons from the post-invasion phase that we hope will be set out by the Iraq inquiry. There should be no doubt that the planning for the occupation was between inadequate and non-existent; that the American and British authorities, military and civil, had next to no idea of the sectarian, cultural and political tensions that rendered Iraq so fissile.

This newspaper has led the way in reporting the aspects of the British management of south Iraq from which lessons should be learned. We told first of the death of Baha Mousa, the hotel receptionist who has come to symbolise the counterproductive brutality of a few soldiers who did the enemy's work for them. We were first with the real story of the deal that was done in late 2007 to pull troops back to barracks and let the militia run Basra – from which the city had to be rescued by Iraq's national army, with the help of US and UK forces in the "Charge of the Knights" operation. The Iraq inquiry heard some evidence about that last week and will hear more this week – though some of it will be in secret, which is disturbing, as the case against openness has not been made.

So, do not expect too much of Mr Campbell this week, or of Mr Blair. But if Sir John could set out a more robust rulebook for ensuring military intervention is justified and likely to stabilise rather than destabilise a society, then what can be the noble principle of liberal interventionism might be rescued from the catastrophes of the recent past.