The invasion of Iraq was not only illegal under international law but also established a very dangerous security precedent for the United States and the international community. International law governing the use of force was in fact derived from this very concern, having been established to prevent the brazen acquisition of colonial markets, which drew the international community into two world wars resulting in the death of tens of millions of people.

The invasion of Iraq raises the question, if the United States government is so heedless of international law that it can choose to invade a country without intelligence justifying the right to exercise military force in pre-emptive self-defense, is any state safe? Can the Iranian government safely assume the United States will not attack it without justification under international law? Can the Russian or Chinese government assume that its vital energy supplies will not be disrupted by such an unprovoked U.S. attack on Iran? Such security scenarios are exactly what international law was designed to prevent from happening. The sanctity of international law, however, has been violated by the most powerful nation in the world. At what cost?

http://nlg.org/resources/iraqfactsheet.pdf