Gwyn Prins: The man is a tyrant whose time has come. The war must be fought

The case against Saddam Hussein is stronger than the Prime Minister makes out

Why did Tony Blair stake high on his weaker card? Yes, he genially mugged or outflanked his more obvious critics on Iraq last week, even if his celebrated dossier added little new to public knowledge about Saddam's weapons of mass destruction. The requirement was to demonstrate a real and present danger. What was weak is that no immediate threat from the Iraqis was shown, although it was darkly hinted at. It certainly made no case that explains why, on the evidence, the coming war to remove Saddam Hussein was not unleashed months or years ago.

The Americans are also on shaky ground, as the newly published US National Security Strategy reveals. There, buried in chapter five, is the suggestion that we may claim to be covered by the "established" right to pre-empt imminent attack (which is not quite as matter-of-fact in my understanding of international law as that passage suggests). It also suggests pre-emption will be triggered on the strategy's tests for "rogue states" such as their ownership of, or ambition to hold, weapons of mass destruction. This is a hard course for our man Sir Jeremy Greenstock to steer successfully at the United Nations. So the puzzle remains. Why? Why do it this way when there is a stronger case? And one closer to the Prime Minister's heart?

Tony Blair is an odd cove among politicians, because the source of his inner driving energy plainly has more to do with his moral passions than with conventional greasy pole ambitions. It may explain his evident distaste for party politics and politicians – and for debate with them – his ruthlessness, his cold-eyed yet jovial slipperiness on most domestic issues and his unusual courage in foreign affairs.

The Kosovars are right to name a street after Blair: he put steel into the vacillating Clintonistas by betting the British Army, if it was needed, to liberate them. He saved the UN from falling flat on its face in Sierra Leone, and willed a magnificent rescue of the Sierra Leonians. His personal commitment underwrote the substantial and successful British effort – diplomatic, military and, especially, in intelligence – that helped to shape the linked American military and German diplomatic campaigns that made the first two phases of the Afghan war such stunning successes.

What a pity, then, that he did not develop the essential case for removing Saddam Hussein. That was the moment to employ his engaging off-the-cuff, from-the-heart register to focus upon paragraph eight of the Iraq dossier: to explain that, at a certain point, tyrants lose their moral right to rule. Pathological governors who deliberately starve, torture or bomb with chemical weapons their own people, as Saddam did the Iraqi Kurds of Halabja in March 1988, and the Marsh Arabs later, cannot hide behind the UN Charter's non-intervention clause.

Blair might have added that when all other means are exhausted, the citizen is justified in rebelling. Nelson Mandela pointed this out with reference to the suffering people of Zimbabwe, just as Mugabe's aggression accelerated. It is this right that lays a duty of support upon those of us who rejoice in the rise and rise of human rights: this most positive of trends in global politics since the end of the Cold War. But Blair didn't.

"Regime change" is not a perverse preoccupation of ugly Americans. The UN's International Commission on Intervention and State Sovereignty earlier this year provided detailed guidance to show where the "high and narrow" threshold of "a shock to the conscience of mankind" is crossed, and what to do.

There are voices raised against this coming war, as yesterday's march for peace held in London forcefully emphasised. But in last week's debate, held after the recall of Parliament, the former Labour minister George Foulkes castigated those whose views resonate with anti-Americanism. Some are beyond reasonable discussion, but there are grounds of objection that can be debated. Other critics of intervention in Iraq fret about the adequacy of any mandate; many bishops, for instance, fear that this will not be a just war. Then there is a third concern: that, even if you accept that there is a defensible legal mandate, there is too great a risk of catalytic breakdown in the Middle East.

As to the arguments for and against the war, three essential tests can be used to score their qualities. Assuming the exhaustion of all other means than force, is "regime change" desirable? Is it properly mandated? Will it work?

I have already explained why regime change in Iraq is desirable. And it is properly mandated, yes. Solidly, and with two feet. One stands upon the UN Genocide Convention in respect of the destruction of the Marsh Arabs and the Kurds. The other relates to the completion of the unfinished war of 1990. So the action is sustained by extant mandates – UN Security Council resolutions 660-678. Those enabled the coalition to liberate Kuwait specifically and, generally, to restore international peace and security to the Middle East region. Bush senior is greatly to blame for stopping the Gulf War. Saddam is patently still in default, and has form. A fresh resolution would be nice, but is not necessary. Unlike milk, mandates do not go sour.

Then there is a broader demand; some say there should be popular assent for action against Iraq. But this is flimsy. It is, in any case, too hard to gauge, given that after recent constitutional sabotage parliamentary debate no longer has the same standing.

Will it work? This matters a great deal to the "just war" critics. My view on the military campaign is that, provided the operation is supplied with overwhelming mass, the defeat of the rump of Saddam's army should not be technically too challenging. Proportionate and discriminate action, with minimum risk to our forces, can then occur. Lord Guthrie, the former chief of the defence staff, pointed this out during the Lords' debate.

The real danger lies in how we handle the second phase of intervention. Here there are grounds for concern. After the end of Saddam's regime a proper military government of occupation, on the model of that after the defeat of the Third Reich, will be required. It will need to provide a protected space for civil society to begin to develop: no mean task after the betrayal of the Kurds and Marsh Arabs by Bush senior in 1990 and the state terror of the Saddam regime. Its mission will be encompassing, but also politically passive – as was achieved on a smaller scale by General McColl in Afghanistan.

Should Bush junior's people be tempted to lever the victory, either by using a new Iraqi base to meddle in Saudi or to turn on Iran – the other regional party in the incongruous, ignorant and deadly axis of evil – then we would finish up with a result that yielded more harm than benefit. In collapsing the region's houses of cards, the Afghan victory, already unravelling through insufficient attention and investment, could be lost too; and the bishops' objections would gain substance. Neither of these need result; but both could, if America botches the peace.

This message, above all, is the one we need Mr Blair to impress upon Mr Bush. As an old imperial power to a new one: Greece to his Rome.

Gwyn Prins is Alliance Research Professor at the London School of Economics and Columbia University, New York; his book, 'The Heart of War: On Power, Conflict and Obligation in the Twenty-First Century', has just been published by Routledge