“The just war tradition… originated in the Catholic Church during the fourth century CE.”
“Ah ha!” I thought, demonic, amateur-historian’s gleam in my eye. “What major, MAJOR event happened in fourth century Christianity that might have necessitated creation of a “Just War” theology? Why that’s when a conquering emperor adopted as his state religion a multifarious, once-Jewish sect grounded in egalitarian community, extreme social justice and absolute, radical abandonment of violence for nonresistance.”
In other words, that’s when Constantine made Christianity the religion of the Roman Empire, thus originating Christian orthodoxy. This included requiring all his soldiers to join the church. His… soldiers? Join a religion committed to nonviolence?
Although his mom was Christian, Constantine himself didn’t seem to have been much of a believer. (For instance, he refused to be baptized until he was about to die.) It’s possible that, like a good pagan, he simply respected all possible gods. Once the Christian god helped him win the battle that made him emperor, he merely elevated that god’s worship. He didn’t seem to care about the details of the theology. He only demanded that there be one, unified set of beliefs, an “orthodoxy,” to be enforced by imperial law.
Except… As emperor of a huge, imperialist empire, he needed police, criminal prosecution, jails, capitol punishment, autocratic governors, laws that kept the rich rich and the poor poor - with ordered trade amongst them, and – most importantly – war. But these were aspects of social organization that Christians had traditionally stood against… (Although many offshoots had already moved from their radical roots into typical, patriarchal church organization - as witnessed by the spreading suppression of women within a century of Jesus’ death.)
Ergo the need to justify war.
As the teachings of Jesus were abandoned, a Christian “Just War theology” was born.
Footnote: There was then no “Catholic Church” (as in “Roman Catholic Church”) such as exists today. In the fourth century, members of the newly established, state church fought long and hard over exactly which of a multitude of beliefs would be called orthodox and which condemned as heresy. The center of church authority was also soon to reside, with the center of imperial power, in Constantinople. Although full of factions, orthodox Christianity had not yet split between Eastern Orthodox and Western Orthodox. …And there were lots of Popes.