Duhaime's Law Dictionary

Casus Foederis Definition:

Latin: treaty event.

Usually in reference to the triggering event in a treaty which causes the signatories to an alliance to deliver upon their commitment, or the treaty to break.

Oppenheim wrote:

"Casus foederis is the event upon the occurrence of which it becomes the duty of one of the allies to render the promised assistance to the other. Thus, in the case of a defensive alliance, the casus foederis occurs when war is declared or commenced against one of the allies. Treaties of alliance very often define precisely the event which shall be regarded as the casus foederis."


  • Oppenheim, L., International Law: A Treatise, 8th Ed., Vol. 1, edited by H. Lauterpacht (New York: Longmans, Green & Co., 1955), page 963-964.

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