The origin of the just war tradition is rooted in Augustine (AD 354-430), moves into the medieval period with the Catholic monks Gratian (“the father of canon law”) and Thomas Aquinas, then into the Reformation period with the stalwart reformers, Martin Luther and John Calvin.  Luther was the first major figure in the just war tradition to denounce religious inspired warfare.  While up to this point the just war arguments were largely rooted in Scripture and addressed to man in the context of sin and salvation, a shift in emphasis toward natural law took place with Franciso de Vitoria (1492-1546), Franciso Suarez (1548-1617), and Hugo Grotius (1583-1645).  John Locke (1632-1704) likewise relied on natural law but extended it to include “a distinctly secular set of goods: political liberty, the right to property, and the hope of material progress” (Charles and Corey 2018).

A thorough treatment of the Christian tradition of just war according to Augustine and Aquinas, which he refers to as “virtue-ethical criteria” in contradistinction to the modern just war theories which tend to focus more on the ease of application of in bello principles, is provided by Nico Vorster (2015).  His research leads him to conclude with nine principles.  I have simplified Vorster’s list by removing repetitions and combining closely related principles, reducing it to 6 principles.

Last resort.  A shared humanity, benevolence, and the brutality of war require that war be declared “as a last resort after all other peaceful remedies such as negotiation, mediation, arbitration and recourse to international tribunals have been sought” (Vorster 2015).  Charles and Corey note that this is not to be taken too strictly, otherwise one would quickly find themselves in a position of ad infinitum; for there is virtually always something else that could be tried or tried once again (2018).

Just cause.  In reference to Aquinas, there must have been some wrong committed worthy of a war being waged.  Pride, glory, or economic gains are not just reasons for waging war.  As Augustine put it in City of God, “For it is the injustice of the opposing side that lays on the wise man the duty of waging wars” (2003).

Legitimate authority.  The principle of legitimate authority can be found in the classic text on government in the Bible, Romans 13, wherein the apostle Paul tells us that “the governing authorities” are “established by God” and they bear “the sword” as “an avenger who brings wrath on the one who practices evil” (NASB ’95).  In other words, the sword is not placed in all hands, but in the hands of a particular institution.

Pre-emptive, not preventive.  Based on Augustine’s treatment of this subject, Vorster defines the difference between pre-emptive and preventive killing as follows: “preventive killing constitutes an anticipatory form of self-defence against an act that may or may not happen, while pre-emptive self-defence occurs when a person’s life is imminently in danger.”  In the case of preventive killing, a greater evil is enacted in order to prevent a counterfactual.  Augustine used an illustration demonstrating a greater evil (murder) to prevent a lesser evil (beating).  The reverse is true in the case of pre-emptive killing, thereby making it a just enactment.  Along these lines, Luther remarked that “where an injustice cannot be punished without a greater injustice, [a ruler] should not insist on his rights, however just his cause” (Charles and Corey 2018).

Peace and order as its goal, or a proper intention.  War may be fierce and chaotic, but it’s ultimate goal should be the restoration of peace, justice, and order in society.  Codevilla (2014) communicates that a nation who does not have peace and order as its just end for war will not peace, and will therefore be in a perpetual state of (unjust) war, resulting in the unnecessary destruction of lives.  Augustine believed that being a peacemaker sometimes means engaging in fighting, for “war is waged in order to attain peace; [and] through your victory you might bring those whom you defeat to the advantages of peace” (Charles and Corey 2018).

Proportionality and Discrimination.  We may think of this principle as being against over-kill.  More damage does not need to be inflicted than is necessary for the achievement of the war’s just end.  This point is closely related to discriminating against combatants and non-combatants (or civilians).  “Separating the innocent from the guilty is the object of judgment, the intention that defines it” (O’Donovan 2003).  Speaking for the whole of the just war tradition, Grotius notes the error in conceiving the enemy as a single body.  C. S. Lewis also expressed disagreement with the lack of concern for this principle exhibited at the bombing of Hiroshima (Ibid.).  In short, war does not free us up to treat others, even our enemies, in an inhumane manner.

Charles and Corey (2018) note five principles of the just war tradition that fall under jus ad bellum: “1) legitimate authority, 2) just cause, 3) right intention, 4) likelihood of success and of doing more good than harm, and 5) last resort.”  The only one not contained in Vorster is the likelihood of success, although the point on doing more good than harm is contained in Vorster.


Augustine. 1972. City of God, trans. Bettenson, Henry. New York: Penguin Books

Charles, J. Daryl and David D. Corey. 2018. The Just War Tradition: An Introduction. Delaware: ISI Books.

Codevilla, Angelo M. 2014. To Make and Keep Peace Among Ourselves and with All Nations. California: Hoover Institution Press.

O’Donovan, Oliver. 2003. The Just War Revisited. United Kingdom: Cambridge University Press.

Vorster, Nico. 2015. “Just War and Virtue: Revisiting Augustine and Thomas Aquinas,” South African Journal of Philosophy, 24(1): 55-68.