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What is a Just War?

In light of recent events in Europe, it’s a good idea to examine the principles of Just War theory in the Christian tradition. Just War theory is not specific to Christians, indeed Cicero of Ancient Rome is in many ways the father of the Just War tradition. That being said, major work on Just War theory has been done by Christians, and more specifically Catholic figures in the West, so it is to them we will mostly turn in this article. It must also be noted that the modern era has developed weapons and war time technology far beyond that which Augustine and Aquinas had any inkling of. How one wages war has fundamentally changed and that is its own discussion. Here, we’ll table that discussion and instead focus on the principles of Catholic Just War teaching.


Currently, the Roman Catholic Church lays out these conditions for a war to be considered legitimate, or just:

  • The damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain

  • All other means of putting an end to it must have been shown to be impractical or ineffective

  • There must be serious prospects of success

  • The use of arms must not produce evils and disorders graver than the evil to be eliminated

All together, these are the benchmarks, so to speak, of Just War. Underlying all of the benchmarks is the virtue of Justice which, as Cicero remarks in De Officiis, “is that no man should harm another unless he has been provoked by injustice. ”[1] A person ought not harm another unless he has been harmed first. There are limits to this response “probably to the point where the man who did harm repents his injustice.”[2] War, therefore, is the public response to unjust treatment from another nation or power. That does not mean, however, that war is always the correct response. Cicero, properly understood, makes it exceptionally clear that “a war can only be just if undertaken as a consequence of an unjust act. Thus, war of aggression [3] is always unjust.”[4] Likewise, the Catholic Church clings to this principle to this day.