Intervention

The interference by one state in the internal or external affairs of another state. It could refer to  interference in a state’s freedom to choose its political, economic, social, or cultural path, or to interference in the formulation of its foreign policy. Intervention may take the form of military action, or it may be undertaken through the exertion of economic or political pressures. However, it will be unlawful under international law if such interference is achieved through the use of force, be it in the direct form of military action or in the indirect form of support for subversive or terrorist armed activities within another state.  

If third States and/or international organizations intervene militarily in ongoing non-international armed conflicts in support of the non-State armed group party to the conflict, this internationalizes the conflict.

See Classification of conflicts; Internationalized internal armed conflicts;