From Christopher Hitchens
Essentially, there are four such criteria. One is genocide, which, according to the signatories of the Genocide Convention (the United States is one), necessitates immediate action either to prevent or to punish the perpetrators. Another is aggression against the sovereignty of neighboring states, including occupation of their territory. A third is hospitality for, or encouragement of, international terrorist groups, and a fourth is violations of the Nonproliferation Treaty or of U.N. resolutions governing weapons of mass destruction.