Casus Foederis

Casus Foederis 

a term used in diplomacy to indicate the situation or fact that will activate a treaty alliance to render mutual assistance. Contemporary international law gives every state the right to individual or collective self-defense (collective self-defense is understood to be assistance rendered by one state to another to resist armed aggression or the threat of aggression). Collective self-defense can be exercised by states without any preliminary formal agreement, or it can be based on special treaties of alliance that usually give the exact terms of the obligation to render mutual help and also specify the casus foederis.