I promised to suggest a definition. It is a very modest one, simply trying to apply the language of article 51 of the Charter about self-defense to the problem of military intervention. It might be desirable to have at least a rule which would say: "No military invention by one state in the internal armed conflicts in another state is permissible except in an extreme emergency requiring instant response and subject to imimediate termination of such emergency action on the request of the United Nations or an appropriate regional organization."
Louis B. Sohn,
Gradations of Intervention in Internal Conflicts
Available at: https://digitalcommons.law.uga.edu/fac_artchop/183