Many people, including Slate's Robert Wright, have suggested that a pre-emptive attack on Iraq would violate "international law." What is international law anyway?
International law encompasses several things, including treaties, which Article VI of the U.S. Constitution makes "the supreme law of the land"; customary law, which refers to rules that all states follow and expect to be upheld; and a web of organizations such as the United Nations, the World Court, the World Bank, the Organization of American States, the African Union, etc.
What specific agreement do doves think the United States would be violating by invading Iraq? Article 2, Section 4, of the U.N. Charter, which forbids "the threat or use of force against the territorial integrity or political independence of any state." Article 51 of the charter reserves for each nation the right of self-defense against an armed attack, but doves don't think this justifies a U.S. invasion.
Say the U.S. invasion of Iraq does violate the U.N. Charter, but we decide to invade Iraq anyway. Then what happens? Probably nothing. There's no international police force going around arresting states that are in violation of international law. A state can be brought before the World Court, but only if it agrees to be or has signed a treaty referring certain disputes to the court. The court can impose damages but can't collect them, which would require action by the U.N. Security Council, of which the United States is a member.
So why do states bother to follow international law? For one thing, they want the benefits as well as the costs, and if they renege on their obligations under an agreement, their partners often will, too. International law clarifies standards and allows for diplomatic horse-trading. But most of all, international law bestows transparency and legitimacy, and states usually follow it because, to some extent, they know their reputation depends on it.
Explainer thanks Ruth Wedgwood and Harold Koh of Yale.