In the 1960s, the White House coined the term "peace offensive" as a euphemism for its war on Vietnam. Today, by "regime change" in Iraq, it means killing Saddam Hussein. The case against him does not hinge upon his breaches of UN resolutions, which have been condoned by most members of the Security Council. His malignancy is to be inferred from the brutality of his dictatorship over his own people. There is a precise word for what parliament will soon debate, and it is not regime change. It is tyrannicide.
The last time that parliament formally approved tyrannicide - indeed, defined it as a duty rather than an option - was in 1649, when the rump parliament had Charles I executed as a tyrant for murdering his own people, committing war crimes and behaving like an absolute dictator. Milton (who served Cromwell in the office of Alastair Campbell) argued that killing a ruler who turns on his own subjects is moral, as well as poetic, justice. Although this cut little ice with the public (who preferred the sentimental royalist spin about Charles the holy martyr), the puritan justification for tyrannicide remains. Who now criticises von Stauffenberg (other than for ineptitude) in his attempt to blow up Hitler, or castigates the Romanians who killed Ceausescu?
But is this just "cruel necessity" - as Cromwell apocryphally muttered over the king in his coffin? Necessity may know no law, but it is the nature of ordered society to find some legal basis for lethal acts, however utilitarian the purpose they serve. Just as we look to domestic courts to find justifications rather than excuses for doctors who turn off life-support machines, so internationally we require precedent or authority for the use of force against an independent and sovereign state.
In Kosovo, despite the absence of any Security Council mandate, Nato discovered in international law a "right of humanitarian intervention". To justify its attack on Afghanistan in response to 11 September, Washington invoked the right of self-defence, expressly preserved in Article 51 of the UN Charter. If the Security Council does not authorise an attack on Iraq (that is, if Russia or China casts a "superpower veto"), the Bush administration will rely on the so-called right of "anticipatory self-defence" - summed up by the phrase "pre-emptive strike".
But the problem with legitimacy, as opposed to necessity, is that it requires reference to some principle, or at least to a precedent. The right of humanitarian intervention, for example, can be traced back to British naval operations to end the slave trade. Self-defence, however, has a classic definition, formulated by the US secretary of state Daniel Webster after the British claimed it in justification of their attacks in 1837 on Americans who harboured Canadian rebels. It legitimates only an immediate response to an actual act of aggression; it certainly does not sanction "anticipatory" or "pre-emptive" use of force. In the example commonly given to law students, the US would in 1941 have been entitled to sink the Japanese fleet the moment it was ordered to set sail for Pearl Harbor, but not to strike it "pre-emptively" out of suspicion that the order might be given.
So the UN Charter's right of self-defence does not legitimate any US attack on Iraq unsanctioned by the Security Council, unless and until Saddam actually acquires nuclear weapons and makes preparations to use them against the US. There is no fact likely to emerge from Tony Blair's dossier to show that Iraq is planning to attack the US or the UK: it will show Saddam to be evil but not suicidal.
These limitations on the right of self-defence may seem irksome, but they have the benefit of disallowing an entirely subjective judgement by the self-defender bent on launching a pre-emptive strike before there is anything to pre-empt. It was Goering's argument at Nuremberg that Germany alone could decide when to go to war in self-defence, and it was rejected by the court: "whether action taken under the claim of self-defence was in fact aggressive or defensive must ultimately be subject to investigation and adjudication if the international law is to be enforced". So this "right" of anticipatory self-defence without Security Council mandate does not exist; if it did, it would be carte-blanche for aggression.
But the limitations on self-defence may be counterbalanced by the development of international human rights rules - in particular, the doctrine that breaches of sovereignty (and inroads on sovereign immunity) may be justified in order to stop or to punish the commission of crimes against humanity. This was the basis for humanitarian intervention to stop ethnic cleansing in Kosovo. It also serves (more appropriately than the claim of US self-defence) to sanction the US overthrow of the Taliban government, which plainly aided and abetted al-Qaeda's campaign of racist attacks against Americans (and anyone else who got in the way) culminating in 11 September. Can this principle also provide a warrant for removal of Saddam Hussein - with or without UN approval?
The answer is yes - up to a point. Turn to the Genocide Convention of 1948, which imposes a duty on parties to punish the leaders of states that engage in ethnic mass murder, and expressly envisages the establishment of an international court to try them. As Tony Blair, quoting Amnesty International, forcefully reminded the TUC, Saddam's persecution of the Kurds in the 1980s left more than 100,000 dead. It culminated in the mustard gas massacre at Halabja in 1988, with 5,000 dead and 9,000 wounded. This is clear evidence of genocide and it was a crime against humanity at the time it was committed. So retrospective prosecution before a special court set up under the convention would be entirely in order. America's opposition to the International Criminal Court does not extend to special tribunals, and it is actually a party to the Genocide Convention - President Reagan ratified it hastily in 1986, in response to criticisms of his visit to Bitburg cemetery with its SS graves.
The practical merit of a special genocide court would be to provide somewhere to put Saddam if he was captured alive. Suppose the US invades Iraq (with or without Security Council approval) and the man comes out of a bunker with his hands up. Is he then to be exiled to St Helena, like Napoleon, or to its current equivalent - the legal limbo-land of Guantanamo Bay? Is he to be invited to form a political party and to stand in UN-sponsored democratic elections?
If the Security Council falters, it follows that an operation to "snatch" Saddam could legitimately be mounted by any party to the Genocide Convention. Whether it would succeed is another matter (there are reported to be a dozen Saddam Hussein look-alikes in Baghdad palaces and command bunkers, awaiting just such an eventuality). And for all the importance of enforcing the Genocide Convention, however belatedly, it must be doubted whether the principle of proportionality could justify an all-out war, costing many innocent lives, to put a dictator in the dock. (The Serb people, by continuing to support Milosevic, took the risk of reprisals for his breaches of international law: the Iraqi people have had no such choice.)
If the Genocide Convention is thought inadequate to justify a unilateral invasion of Iraq, what alternative legal basis can be found? It is not an international crime to possess nuclear or chemical weapons. The only answer, therefore, is a new international right to act against tyranny. That was certainly the message of President Bush's address to the UN. He spoke in terms that Milton and Cromwell would have approved, urging that "liberty for the Iraqi people is a great moral cause". He dilated upon evidence, culled from Amnesty and Human Rights Watch reports, of the all-pervasive repression of the Iraqi people - arbitrary arrests, secret detentions, summary executions, torture by electric shock, mutilation and rape. In other words, human rights violations have reached a point that justify international action to overthrow the regime, whatever UN inspections may reveal about its development of weapons of mass destruction. This is the case for assisted tyrannicide.
In 1649, parliament and its lawyers had no difficulty in defining the crime of tyranny, but even they realised the need for a trial before execution. We now have an International Criminal Court to punish "crimes against humanity", but this offence does not include running a despotic government that commits widespread human rights violations.
There is much to be said for the creation of a new international crime of tyranny but nothing to be said for installing the American president as its judge, jury and executioner. It would require an independent human rights court to examine the evidence, before a verdict that would provide a legal justification for forcible regime change. There is little doubt that Saddam would be convicted. But so, too, would be our new friend Colonel Gaddafi, along with the juntas in Burma and North Korea. The rulers of Turkmenistan and Liberia might not be able to put up much of a defence, and it could not be long before someone contemplates a prosecution against the royal house of Saud.
The ultimate goal of the human rights movement should be to eliminate all rulers who abuse their power by comprehensively violating the liberties of their subjects. But this calls for an international convention against tyranny rather than a declaration of war against the first tyrant to stiff the US. As our parliament discovered in 1660, the problem with tyrannicide is that it can look to later generations very much like martyrcide, and provide the argument for a restoration.
Geoffrey Robertson, QC is the author of Crimes Against Humanity: the struggle for global justice, second edition published this month by Penguin