What is “casus belli’?

April 21, 2022


2 min read

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Casus Belli is a Latin expression that translates as ‘occasion of war’, that refers to the events or acts that nations use to justify going to war. The origins of the phrase can be found in the writings of legal and political theorists such as Hugo Grotius, Cornelius van Bynkershoek and Jean-Jacques Burlamaqui. They and other writers in the 17th and 18th centuries developed the concept of ‘Just War’ and the foundations of International Law. The concept goes back even further to Thucydides, who wrote of ‘Proschema’ in the 5th Century BC, which are the stated reasons for waging a war.


When going to war, a nation’s government typically lays out its reasons for waging war, the way in which it intends to carry out the war and gives some methodology of de-escalation. This is done to show that the severity and impact of such a declaration is understood, and that the war is lawfully justified. 

Modern International Law sees only three valid justifications for going to war: in cases of self-defence, in the defence of an ally as required by the terms of a treaty or if approval has been granted by the United Nations. 

Casus Belli have been notable in several major conflicts. However, the Casus Belli will often only be a flashpoint that sets off a conflict that has already been building. For example, the Confederate attack on Fort Sumter served as the official Casus Belli for the American Civil War. But there was already simmering resentment and aggression between the northern and southern states. Casus Belli have in the past been such ambiguous and unidentifiable things as President Bush citing “democracy” as a casus belli for the Invasion of Panama in 1989.


Casus Belli, and the other associated terms mentioned previously, are important for budding international criminal or humanitarian lawyers to know and understand. The obvious question lurking in the background of these legal principles is, does the law really matter at all in these situations? Increasingly, nations no longer declare their reason for going to war due to the nature of war changing. Often countries will not be seeking to fight other countries, but instead they launch attacks in search of terrorist groups or other such organisations. Similarly, many argue that even when nations do declare legitimate casus belli, these are merely a disguise for the real motivations for going to war. Therefore, it’s always important to evaluate the stated casus belli in the wider political, social and legal context.

Report written by Hari Majumdar

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