casus belli
The event a side points to as its justification for going to war.
“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.”
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.
Hannibal, Pyrrhus, and the strategy Rome used on everyone else.
- 01
Hannibal at the gates (idiom)
A threat so close and so serious that everything else stops mattering.
Hannibal in Italy, 218–203 BC.
- 02
a Pyrrhic victory (idiom)
A win that costs so much it feels almost like a defeat.
Pyrrhus beats Rome and loses anyway.
- 03
divide and conquer (idiom)
To keep opponents from uniting so that each can be dealt with separately.
How Rome handled the defeated.
- 04
fortune favours the bold (idiom)
Luck tends to go to the person willing to take the risk.
Virgil, Aeneid X.
- 05
si vis pacem, para bellum (idiom)
If you want peace, prepare for war — the argument that strength is what prevents attack.
Vegetius on why armies train.
- 06
semper fidelis (idiom)
Always faithful — a pledge of loyalty that does not expire.
The soldier's oath.
Last updated August 3, 2026