"Medieval thinking on war, including that of Thomas Aquinas, was based on St Augustine's theory of the just war as laid out in On the City of God (5th century AD).
St. Augustine saw war as a means to deal with sin. War was a judicial action in which the people fighting were, in one way or another, righting a wrong. As Agustine put it originally, "justa bella ulciscuntur injurias" (just wars avenge injuries) also means that the those who who wages wage war play the role of God's scourge and that this action, inspired by love, is beneficial even for him against whom it is directed. The Augustinian attitude was that you have to show your love any way you can and war was simply a large scale application of the death penalty to people who had earned it.
In more practical terms, a king might say to another that "your dynasty is wicked and I will invade you and wipe you out to save your people from your wickedness." What they are really saying is that "your lands are in disarray and I'm going to attack you because you're not strong enough to defend yourself."…"
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According to Augustine, you can only take up the sword if you are the injured party, or if you perceive an injustice that needs redressing (such as the occupation of the Holy Land by the infidels). Thus, to this day, an aggressor almost always tries to come up with some injury to himself to justify his own invasion
