The cost of intervention is lower than the cost of inactivity. Sometimes, as in Afghanistan and the former Yugoslavia, the situation will become so bad that US military intervention is necessary - this is hugely costly compared to funding preventative action through the United Nations. The role of failed states as reservoirs from which refugees, narcotics, terrorism, illegal diamonds, etc. are exported means that the USA already spends many billions of dollars a year in dealing with the mess they create. Finally, there is an opportunity cost of lost trade and investment which applies to the developing world and developed economies alike (e.g. the benefits to the US of trade with oil-rich Angola, Sudan and Congo).