No contemporary problem of world order is more troublesome for an international lawyer than the analysis of the international law of “internal war.” A war is usefully classified as internal when violence takes place primarily within a single political entity, regardless of foreign support for the contending factions. The insurgents who won the American Revolution were heavily supported by French arms. Wars of national liberation are not new, nor is external support for an incumbent regime. But considerable historical experience with foreign intervention in internal wars has not been adequately incorporated into prevailing doctrines of international law. In an age of civil turbulence and nuclear risk, the requirements of world order make imperative the effort to overcome the consequent confusion.

The central issue is whether an externally abetted internal war belongs in either traditional legal category of war – “civil” or “international.” Four sub-inquiries are relevant. What are the legal restraints, if any, upon national discretion to treat a particular internal war as an international war? What rules and procedures are available to determine whether foreign participation in an internal war constitutes “military assistance,” “intervention,” “aggression,” or “an armed attack”? What responses are permissible by the victim of “aggression” or “an armed attack”? Finally, what should be the roles of national, regional, and global actors in interpreting and applying the relevant rules?

If the internal war is regarded as a “civil” war, then the legally permitted response to intervention is restricted to counter-intervention; an intervening nation whose own territory is not the scene of conflict may not attack the territory of a state intervening on the other side. If foreign intervention were held to convert an “internal” war into an “international” war, the intervention could be regarded as an armed attack that would justify action in self-defense proportionate to the aggression. The victim of aggression is entitled, if necessary, to attack the territory of the aggressor, expanding the arena of violence to more than a single political entity. Given the commitment of international law to limiting the scope, duration, and intensity of warfare, it would appear desirable severely to restrict, or perhaps to deny altogether, the discretion of nations to convert an internal war into an international war by characterizing external participation as “aggression” rather than as “intervention.”