A (; ) is an act or an event that either provokes or is used to justify a war. A casus belli involves direct offenses or threats against the nation declaring the war, whereas a involves offenses or threats against its ally—usually one bound by a mutual defense pact. Either may be considered an A declaration of war usually contains a description of the casus belli that has led the party in question to declare war on another party.
Terminology
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).
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 (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.
Proschema (plural proschemata) is the equivalent Greek term, first popularized by Thucydides in his History of the Peloponnesian War. The proschemata are the stated reasons for waging war, which may or may not be the same as the real reasons, which Thucydides called prophasis (). 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 (as opposed to descriptions of reasonable, empirical causes for fear).
Reasons for use
Countries need a public justification for attacking another country, both to galvanize internal support for the war and to gain the support of potential allies.
In the post–World War II era, the UN Charter prohibits signatory countries from engaging in war except: 1) as a means of defending themselves—or an ally where treaty obligations require it—against aggression; 2) unless 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.
Categorisation
Braumoeller (2019) stated: "However idiosyncratic the casus belli may seem, however, there generally is one ... The issues that prompt most wars fit fairly well into one of a fairly manageable number of categories." He broadly summarised classical issues as territory, the creation or dissolution of countries, the defence of the integrity of countries, dynastic succession, and the defence of co-religionists or co-nationals. He pointed out that in the modern field of peace and conflict studies, scholars also frequently list causes such as "struggle for power, arms races and conflict spirals, ethnicity and nationalism, domestic political regime type and leadership change, economic interdependence and trade, territory, climate change-induced scarcity, and so on".
In The Causes of War (1972), Australian historian Geoffrey Blainey mentioned general causes such as miscalculation, as well as specific causes such as "Death Watch and Scapegoat Wars", and emphasised the importance of mundane factors such as weather.
Theodore K. Rabb and Robert I. Rotberg explored the roots of major conflicts as a mixture of factors on the international, domestic and individual level in The Origin and Prevention of Major Wars (1989).
Kalevi Holsti catalogued and categorised wars from 1648 to 1989 according to 24 categories of "issues that generated wars".
A casus belli intentionally based on inaccurate facts is known as a pretext.
Historical examples
This section outlines a number of the more famous and/or controversial cases of casus belli which have occurred in modern times.