# casus belli

idiom · CEFR C2 · Formal

Source: https://englishreference.com/idiom/casus-belli
Updated: 2026-08-03

## Meaning

The event a side points to as its justification for going to war.

## Examples

- The attack on the border post became the casus belli.
- Historians still argue about whether the incident was a genuine casus belli or a pretext prepared months in advance.
- The declaration named a single sunk vessel as its casus belli, a framing that let the government present a decade of accumulating grievance as a sudden and unavoidable response.

## Origin

Rome was fastidious about the paperwork of war. A college of priests called the fetiales handled the formalities: a grievance was stated, restitution was demanded, a waiting period ran, and only then could a war be declared and still count as a just one — a bellum iustum. The ceremony ended with a priest hurling a spear into enemy territory. The Latin phrase casus belli, the occasion of war, is not classical. It belongs to the international lawyers of the sixteenth and seventeenth centuries, who built their vocabulary out of Roman legal language while working out when one state may attack another. It stayed in English because the problem never went away. A casus belli is the incident a government holds up as its reason for fighting, which is not always the same thing as the reason it is fighting.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.