Casus Belli

Lore & Background

The term casus belli came into widespread use in Europe in the seventeenth and eighteenth centuries through the writings of Hugo Grotius (1653), Cornelius van Bynkershoek (1707), and Jean-Jacques Burlamaqui (1732), among others, and due to the rise of the political doctrine of jus ad bellum or 'just war theory'. The term is also used informally to refer to any 'just cause' a nation may claim for entering into a conflict. It is used to describe the case for war given before the term came into wide use, and to describe the rationale for military action even without a formal declaration of war (as in: the lead up to the Gulf of Tonkin Resolution).

Reader's Guide

In formally articulating a casus belli, a government typically lays out its reasons for going to war, its intended means of prosecuting the war, and the steps that others might take to dissuade it from going to war. It attempts to demonstrate that it is going to war only as a last resort or plan (ultima ratio) and that it has 'just cause' for doing so. Modern international law recognizes at least three lawful justifications for waging war: self-defense, defense of an ally required by the terms of a treaty, and approval by the United Nations.

In the post–World War II era, the UN Charter prohibits signatory countries from engaging in war except: as a means of defending themselves, or an ally where treaty obligations require it, against aggression; when the UN as a body has given prior approval to the operation. The UN also reserves the right to ask member nations to intervene against non-signatory countries that embark on wars of aggression.

Did You Know?

• Proschema (plural proschemata) is the equivalent Greek term, first popularized by Thucydides in his History of the Peloponnesian War.
• Thucydides argued that the three primary real reasons for waging war are reasonable fear, honor, and interest, while the stated reasons involve appeals to nationalism or fearmongering.
• A casus belli intentionally based on inaccurate facts is known as a pretext.