Phyllis Bennis, author of Understanding the U.S.-Iraq Crisis: A Primer, and John Cavanagh, director of the Institute of Policy Studies, have prepared these Talking Points on the U.S.-Iraq Crisis. From the introduction:

This preventive war (it isn't even preemptive because there is no imminent threat to preempt) is among the most dangerous and reckless actions ever taken by a U.S. president. It isn't the first time the U.S. has launched an unjustified illegal war. But it is the first time such a war has been justified through a "doctrine" of preemptive war that abandons all understandings that war, with all its horrors, can be used only as the last possible resort when a nation's security and survival are threatened.

Some of the points:

The war at home

This war threatens Americans. We are now at greater risk. This war will increase anti-American sentiments around the world, and will serve as a recruiting poster for al-Qaeda and other terrorist organizations....This is an illegitimate war, and stands in violation of the UN Charter and international law. We hold the U.S. accountable for this illegal war.

The War Abroad

This war will be devastating for Iraq and Iraqis. The Pentagon's plan of "shock and awe" to open the main air attack against Baghdad will send 3,000 cruise missiles and precision-guided bombs into a crowded city of 6 million people. That is ten times the number of such bombs used in the entire Gulf War in 1991.
The humanitarian consequences will be severe.

What we should do

We should demand that Congress refuse to pay for waging an illegal war. We should also be clear that the U.S. is accountable for paying the costs of rehabilitating Iraq's war-shattered infrastructure as well as the emergency costs of refugees, food aid, etc. That money should be channelled through the UN humanitarian agencies, not paid to U.S. corporations, especially those (like Halliburton - already offered a $1 billion + contract) with direct links to the Bush administration.

We should urge the General Assembly to convene in emergency session under the terms of the Uniting for Peace precedent at the UN. That allows the Assembly to take up issues ordinarily reserved for the Security Council - such as war in Iraq -- when the Council is paralyzed because of disagreements among the five veto-wielding permanent members. A General Assembly resolution (where there is not veto) could condemn the U.S. war (important for delegitimating future such wars), demand an immediate halt to the war, request an advisory opinion from the International Court of Justice on the legality of the war, or more.