Casus (or code) foederis

Casus foederis (or code foederis or casus fœderis) means:

1. It is derived from the Latin for "case for the alliance". In diplomatic terms, it describes a situation in which the terms of an alliance come into play, such as one nation being attacked by another.

• Sources: https://en.wikipedia.org/wiki/Casus_foederis.