1911 Encyclopædia Britannica/Casus Belli
CASUS BELLI, the technical term for cases in which a state holds itself justified in making war, if a certain course to which it objects is persisted in. Interference with the full exercise of a nation’s rights or independence, an affront to its dignity, an unredressed injury, are instances of casus belli. Most of the new compulsory treaties of arbitration entered into by Great Britain and other states exclude from their application cases affecting the “vital interests” or “national honour” of the contracting states. These may therefore be considered as a sort of definition of casus belli in so far as the high contracting parties to them are concerned.