… Hindsight is a wonderful thing. It means that, while success has many fathers, failure only ever has one.

Needless to say, we historians have the benefit of hindsight, too. In passing judgment, however, we try to focus on what decision-makers knew at the time of their decision. The great merit of the Chilcot Report is that it reconstructs that decision-making process meticulously, and shows where it went wrong.

Thirteen years on, those of us whose warnings were ignored are in a better position to criticize Blair than those who egged him on. Yet my inclination is to defend him. Unlike his turncoat critics, he has at least had the guts to express “sorrow, regret, and apology.”

In his impassioned apologia last week, Blair asked us to put ourselves in his position: “You are seeing the intelligence mount up on WMD. You are doing so in a changed context of mass casualties caused by a new and virulent form of terrorism. You have at least to consider the possibility of a 9/11 here in Britain. And your primary responsibility … is to protect your country.”

The UN was gridlocked, with France and Russia vetoing action that was justified under Resolution 1441. So we joined Bush’s war because — in Blair’s words — “I thought the human cost … of leaving Saddam in power would be worse for Britain and the world.”

I took a different view, as we have seen. But how can I be sure I was right? As Blair pointed out last week, we need to ask the counterfactual question: What if Saddam had been left in power? The alternative future Blair asks us to imagine is not wholly implausible. If the forces assembled in March 2003 had not been used, “sanctions would have swiftly eroded,” the system of inspections would have crumbled, and an “immensely … strengthened” Saddam would have resumed his WMD programs.

Moreover, if Saddam had still been in power in 2011, would there not have been an Arab revolution in Iraq, too? “In that case,” argued Blair, “the nightmare of Syria today would also be happening in Iraq.”

I know, I know. After all that went wrong in Iraq, beginning in March 2003, it is hard indeed to imagine a worse scenario. But this perfectly illustrates why all political lives are doomed to end in failure. For leaders must act on the basis of conjecture as well as of intelligence. In 2003 Tony Blair’s conjecture was that leaving Saddam in power would be worse than overthrowing him. In 2003 most of the press agreed. Today it seems obvious to the journalists who once cheered him on that Blair’s conjecture was wrong. The reality is that we cannot be sure. All we can be is honest with ourselves. Failure, too, has many fathers….