William Saletan makes an excellent point in his assessment of Bush's UN speech -- that the best reason for forceful action against Iraq is that country's utter disregard for U.N. resolutions. The money line:

Saddam's history with the U.N. is a joke. As Bush amply detailed today, Saddam has betrayed pledge after pledge, circumvented sanction after sanction, and defied warning after warning from the U.N. Security Council.

This turns the liberal argument against war on its head. A principled liberal must be prepared to punish those who defect from multilateral norms. There is no question that Iraq has defected from those norms. Comprehensive sanctions are already in place; the last remaining option is the force of arms.

The sanctions provide the other liberal argument to invade. The best (and least biased) study of the sanction's effects on Iraqi children shows that the price has been high. Who's to blame for this? Obviously, Saddam -- he rejected the oil-for-food programme in its first four years of operation. But if Saddam is to blame, the U.S., as the instigator of the sanctions, must share some moral responsibility for loss of life. One way to deal with this would be to create a CoCom-style strategic embargo, but the Bush administration tried and failed to get the Security Council to go along with this before 9/11. The other option is to convert an unjust sanctions regime into a just war. Some military analysts think that, in terms of civilian casualties, military action is more humane than sanctions.

This might be why there has been such vacillation among liberals about Iraq -- because the principles of multilateralism and just war dictate the use of force.

PS: The New Republic points out that these liberal arguments scare hawks because they don't necessarily lead to regime change.