There is an old and not unfamiliar adage that says, ''When the guns roar, the muses are silent,'' and while the legal muses have been unable to prevent the war on Iraq, it is still appropriate to determine whether the conflict conforms to international humanitarian law.

The United States' resort to force might arguably appear justifiable against one of the most murderous and tyrannical regimes. For more than 20 years, Saddam Hussein has sought not only to acquire weapons of mass destruction but has used them. He gassed thousands of his own people at Halabja, while "cleansing" Marsh Arabs after the first Persian Gulf war. He launched two catastrophic wars, sacrificing nearly a million Iraqis and killing or wounding more than a million Iranians. He threatened to incinerate Israel in 1990 and, today, finances suicide bombers.

During the past 12 years, he has violated 16 United Nations resolutions that found him to be in material breach of his disarmament obligations, with Security Council Resolution 1441 being his "last opportunity" to comply. But while he is clearly a war criminal and the personification of evil -- an inviting target for a "just war" -- this does not necessarily authorize the use of force against him, unless it's in accord with the basic principles of international law.

The first principle, grounded in the UN Charter, is the prohibition of the use of force, save two exceptions: the exercise of the right of self-defence in response to an armed attack as mandated under Article 51 of the Charter; and the right of the Security Council, under Chapter 7, to authorize military action. A third, emerging exception -- humanitarian intervention to avert international crimes such as genocide or crimes against humanity -- arguably requires Security Council authorization, which was sought before intervention in Kosovo.

President George W. Bush argues that the United States has a right of pre-emptive self-defence, that in a post-Sept. 11 universe with its convergence of terrorism, weapons of mass destruction and rogue states, awaiting an armed attack can convert the UN Charter into a suicide pact. But even allowing for a more flexible interpretation of the right of self-defence, there must be at least credible evidence of imminence of such an attack, which has not been adduced.

Both Mr. Bush and British Prime Minister Tony Blair also argue that, since Iraq has been in material breach of Security Council resolutions, including 1441, then "serious consequences" ensue, including the use of force. This leads to the second principle: Resolution 1441 as a basic juridical framework for appreciating the legalities.

It appears that 1441 is more cited than understood. Under the resolution, only the Security Council, not a state or coalition of states, can determine whether there has been a material breach, what serious consequences ensue, and if the use of force is justified. The Security Council made no such determination, and a second resolution to that effect did not secure support from even a majority of its members.

A third, related principle is that of the "exhaustion of remedies short of war," where the UN Charter requires that only if other measures have proved to be inadequate can force be authorized. This principle is not unrelated to the law of unintended consequences. War is, at best, unpredictable. This principle is a protective antidote to war's unintended consequences.

The fourth principle -- the invocation of the Kosovo precedent -- is organized around the doctrine of humanitarian intervention. But no Security Council or multilateral support was even sought, let alone sanctioned for this doctrine, as in Kosovo.

A fifth principle -- accountability -- has been ignored thus far. But this is particularly important now that war has begun. International humanitarian law principles are clear in what is permissible regarding conduct during hostilities. The use of weapons in any armed conflict must be proportional to the threat; must respect the principle of civilian immunity; must not cause unnecessary or aggravated suffering to combatants; and must not cause severe and long-term damage to the environment.

Saddam Hussein has violated all these and more, and should have been indicted long ago for his international crimes, an option I recommended even before the first gulf war and which might have pre-empted his continuing criminality.

Finally, we come to the principle of retroactive validity. This is perhaps the most compelling one that may operate in favour of Mr. Bush and his coalition. If the war uncovers weapons of mass destruction, exposes Iraq's deception, and is supported by the Iraqi people, it may well be that the legal niceties surrounding the use of force will be overtaken by post-invasion validation.

It may be regarded as trivial to deal with legal niceties now that war has begun. But it is important to recall that Mr. Bush and Mr. Blair, to their credit, argued the case for the use of force pursuant to international law principles and regarded Resolution 1441 and preceding UN resolutions as the juridical basis underpinning their actions. International law, then, is a relevant framework for assessing the validity of their actions. Now that war has begun, norms of international humanitarian law remain an important benchmark to determine the legality of the conduct of hostilities.

Irwin Cotler, Liberal MP for Mount Royal, is chair of the foreign affairs subcommittee on human rights and international development. He is on leave from McGill University, where he specializes in human-rights law.