It hardly matters who was right and who was wrong about the invasion of Iraq in 2003. The main significance of that decision now is that we British have a greater responsibility to the people of Iraq than we otherwise would. Our government helped to add to their misery over the past 11 years, and it helped to open the door to the disaster that is now unfolding.
As Joan Smith writes today, the debate in Britain about Iraq, Syria and the wider region has been "frustratingly parochial", consisting largely of variations on the theme of "Tony Blair was wrong". So he was, but the question is what should be done now.
The temptation is to say, "Haven't we done enough harm?" and to withdraw from the world. However, The Independent on Sunday was never opposed to all wars; we are opposed to dumb wars, as Barack Obama once said. We support the ideal of liberal interventionism, of the responsibility to protect people from crimes against humanity, even by military force if necessary.
The question facing the international community now is whether there is anything realistic that can be done to protect Iraqis. Their country is heading towards the all-out civil war between Sunni and Shia that seemed to have been avoided in 2008, when a remission of violence allowed the British combat forces to withdraw in 2009 and the Americans to depart in 2011.
The conflict in Iraq has made a fool both of those who got the United States and its allies into it and those who got them out. George Bush, with his boast of "mission accomplished" in May 2003, and President Obama, with his wishful "We're leaving behind a sovereign, stable and self-reliant Iraq" in December 2011. Mr Blair should have stayed out, but, once we were in, Gordon Brown should not have cut and run.
Not even Mr Blair is arguing for regular Western forces to return to Iraq, although John McTernan, his former political secretary, has made that case. Mr Blair, in his long essay published last night, argues for something both more "graduated" and more ambitious. He wants the world's rich democracies to support those forces in the Muslim world who would stand up to religious extremism. He is not wrong, but it seems rather vague and, to the extent that he might be saying something useful, it is less so coming from him.
He is, as it happens, right to note that Nouri al-Maliki, the Iraqi prime minister, bears the most immediate responsibility for the corruption and sectarianism of the state, and for the collapse of the Iraqi army, which ran away from Mosul. But Mr Maliki owes his position to a deal to which Mr Blair was a party as much as to his flawed democratic mandate.
There is, in short, little that outsiders can do. But what little that can be done should be done more intelligently than in the past. If the international community gives "support" to the Iraqi government, it should not take the form of hard currency: the heist in Mosul has already made the Islamic State of Iraq and the Levant (Isis) the richest terrorist group in the world. Supplying arms is no more useful, as the Humvees driven round Mosul by Isis fighters attest. There could be a role for US and British military advisers and trainers, but they were asked to leave as a political gesture by Mr Maliki. As for drones or air strikes, it is hard to imagine that they could do anything but make matters worse.
The best that can be hoped for in the short run is that Mr Maliki realises the mistakes he has made and learns from them, and that he can turn the tide against Isis. But that will not resolve the situation in Syria, or the separate crises in Libya and Egypt, where all options for Western policy seem bad.
Perhaps the limit of the ambition of the new interventionism should be to avoid making matters worse.