Doctrines of what constitutes a just war developed out of Roman law, religious encyclicals, military theory and practice, modern political theory and philosophy, as well as international law and jurisprudence. The roots in Western thought can be traced to classical moral, legal, and historical sources and include the scholastic tradition as well as the medieval notions of chivalry and honor. A critical debate in the United States over participation and conduct in the Vietnam War revived just war theory in the West among secular philosophers and Christian thinkers.

The debate was an attempt to determine whether U.S. participation in the war was unjust in origin or in the way it was being fought. Distinctly different but significant notions of just and unjust war arose in Communist theories on the “people’s” war against fascism in the 1930s and 1940s, “national liberation” struggles against colonial powers after World War II, and “holy war” in modern Islam, for example, during the Iran-Iraq war. “Jihad” has become a cliché for the struggle of radical Muslim forces against mostly Western powers.

The criteria for engaging in a just war, first summed up by Dutch philosopher Hugo Grotius in the seventeenth century and drawing on older, medieval Catholic theologians, consists of seven elements: (1) that there be a just cause; (2) that there is a right authority (legitimate sovereign) to initiate the war; (3) a right intention on the part of the parties using force; (4) that the resort to force be proportional; (5) that force be a last resort; (6) that war is undertaken with peace as its goal (not for its own sake); (7) and that there be a reasonable hope of success.

More recently, the term just war has been largely replaced by the term legitimate use of force. The principles of just war today are contained in the United Nations Charter, which reaffirms the inadmissability of the acquisition of territory by force. Although conquest was legally prohibited in Europe in the mid-nineteenth century, European powers compensated by engaging in conquests abroad. Within Europe there remained a branch of thought, largely Prussian, which continued to argue that the principle of conquest—might is right—provided ample cause for a just war. This school of thought lost all legitimacy as a result of World War II.

Today, a just war is commonly understood to mean one that is fought in self-defense, as authorized in Article 51 of the UN Charter. This is the one principle that has been clearly defined and consistently emphasized throughout the history of just war theory. The United Nations General Assembly also has set out a comprehensive and strict definition of illegal aggression and justified self-defense.

A non-Western, indeed predominantly anti-Western, school of thought on just war thinking—the doctrine of national self-determination—arose out of the struggles for colonial emancipation. The United Nations went some way toward endorsing this doctrine through General Assembly resolutions of the 1960s. The justification for launching military action in the Gulf War, conveyed in the preambles to numerous Security Council resolutions, also relied upon principles of just war as espoused in the UN Charter.

(See crimes against peace; jus ad bellum/jus in bello.)