It is perfectly legal for the US-led coalition to enforce resolution 1441.

Whether force has been used contrary to international humanitarian law can only be evaluated after the event and in light of all the evidence, weighing the military advantage anticipated against the foreseeable harm to civilians, according to the established principles of reasonableness and proportionality. There is no reason to presume that a breach by coalition forces of the humanitarian laws of armed conflict will occur.

Perhaps the most disturbing part of the joint letter is the warning that if Australia dares join the attack on Iraq, it may face a spate of future "war crimes" trials before the International Criminal Court.
The alarm thus sounded appears consistent with the worst fears of critics of the ICC, who argued that such a tribunal would more likely deter democracies acting in defence of international legal standards and human rights than it would a Saddam Hussein.

Finally, we believe that morally it is necessary to consider the costs to be borne in ruling out the use of force. Hussein continues to torment his people, foster international terrorism and develop an illegal arsenal of chemical, biological and nuclear weapons. In addition, the authority of the Security Council under Chapter VII is being damaged.

Twelve years of resolutions and sanctions, and promising dire consequences, have not brought Iraq into compliance. When a policeman refuses to enforce the law, and gives a series of hollow "last chances", there comes a point when citizens and criminals alike lose respect for both him and the law.