Nigel Biggar traces a popular idea back to the fourth century.
In the West, just war thinking first came into prominence in the fourth century with St Augustine, though he drew from ancient Roman sources like Cicero. And the reason it came into being was, for the first time in the fourth and fifth centuries, Christians who up to that point had been on the margins of society suddenly found themselves admitted to positions of public office. And for the first time, Christians had to think about, how do we use public office and in particular how do we square our Christian faith with the use of force? Because in those days, as in these, public office sometimes has to do with the use of force to stop criminal activity or to defend borders. Then Christians had to ask themselves that question.