What is not the decisive issue in evaluating the Iraq invasion and what is

Judging by the dominant reaction of the British press, [the sole function of the Chilcot inquiry into the Iraq war] is to prove what we all know to be true: that the invasion was immoral and Tony Blair is to blame

writes Nigel Biggar in the Financial Times. Currently in the process of writing a book on the ethics of war, he is the regius professor of moral and pastoral theology at the University of Oxford.

The surfeit of moral certainty among the commentators is suspect; the zealous clarity of their moral waters needs muddying.

For sure, the invasion and occupation of Iraq was morally flawed. The US administration's motivation was hubristic and preparation for postwar reconstruction was woefully inadequate. Yet most just wars are flawed. Take the war against Nazi Germany. The RAF's indiscriminate bombing of German cities was largely driven by "Bomber" Harris's vengeful hatred. While the destruction of Hitler's hegemony was very good, the entrenchment of Stalin's was very bad. Any complex human enterprise will involve moral flaws. What needs determining is whether and how these undermine its justice as a whole.

…Arguments about a war's disproportion are often intractable. If one assumes the Iraq war was unjust, then no civilian deaths were worth it. Yet in affirming the justice of the war against Hitler we imply it was worth the deaths of 30m civilians. The loss of 150,000 civilians therefore does not, of itself, make the Iraq war unjust. The invasion would be harder to defend were the country's new regime to fail. But that has not happened yet, and those critics who care more for Iraqis than they hate the former US and UK leaders George W. Bush and Mr Blair will hope it never does.

…If determining the Iraq war's proportionality is difficult, maybe determining its legality is easier. It would seem so … But such condemnations can only be opinions, since international law can be variously interpreted. However, even if we grant that the invasion was illegal, we still have to grapple with the fact that so was Nato's 1999 intervention in Kosovo, which is now widely regarded as legitimate. The implication? That legality is not the final word.

Current international law is morally problematic. … The decisive issue in evaluating the Iraq invasion is not whether it was morally flawed or disproportionate or illegal, but whether it was really necessary to stop or prevent a sufficiently great evil.

No one disputes that Saddam Hussein's regime was grossly atrocious. … Critics of the invasion would presumably not tolerate such a regime in their own backyard; and an effective international policing authority would have changed it. Is the coalition to be condemned for filling the vacuum? Yes, there have been similar vacuums that it (and others) have failed to fill — Rwanda, Zimbabwe, Darfur. But is it not better to be inconsistently responsible than consistently irresponsible?

Now add the concern about weapons of mass destruction. This was sufficiently grave to rouse the UN to litter the period 1991-2003 with 17 resolutions calling on Saddam to disarm permanently. … there was good reason to withhold benefit of doubt and to suppose that it was developing WMDs. It was not just Messrs Bush and Blair who supposed this. So did Jacques Chirac, then French president, and Hans Blix, the UN's chief weapons inspector.

We now know this reasonable supposition was mistaken and that the problem was less urgent than it appeared. But it was still urgent. Saddam was intent on acquiring nuclear weapons and support for containment was dissolving.

Related: Exaggerated claims, substandard research, and a disservice to truth (shookhran to MadiMaxi): ORB's "million Iraqi deaths" survey seriously flawed, new study shows.

“Conflict Deaths in Iraq: A Methodological Critique of the ORB Survey Estimate” by Michael Spagat and Josh Dougherty, just published in Survey Research Methods, describes in detail how the ORB poll is riddled with critical inconsistencies and methodological shortcomings.