Matthew C. Waxman, Adjunct Senior Fellow for Law and Foreign Policy
As a matter of international law, humanitarian intervention—such as the use of military force to protect foreign populations from mass atrocities or gross human rights abuses—is permissible if authorized by the United Nations Security Council (UNSC). Although many Western governments have taken the position that such intervention may in some cases be morally justified even if not authorized by the Security Council, most states and international legal experts do not regard that as lawful.
The main source of international law on this issue is the United Nations Charter, which prohibits the use of military force against or in another state without its consent except when authorized by the UNSC or in self-defense against armed attack. The UNSC has authorized humanitarian interventions in cases such as Somalia and Haiti, but it is often difficult to obtain the necessary votes in the UNSC and to overcome resistance by permanent members Russia and China, which are generally opposed to these actions.
In the 1999 Kosovo crisis, NATO launched military strikes to stop Serbian ethnic cleansing, despite opposition from Russia, China, and many states of the global South. Although many Western states regarded the action as morally legitimate under the circumstances, many other states criticized it as violating the principle of state sovereignty.
In recent years, states have reached general consensus that they have a "Responsibility to Protect" populations from mass atrocities, and that when a government fails in this responsibility towards its own people, international action is appropriate. Many states, however, maintain the position that only the UNSC can authorize armed intervention. I consider these issues in a 2009 Council on Foreign Relations Special Report, Intervention to Stop Genocide and Mass Atrocities: International Norms and U.S. Policy.