For the first 300 years of Christianity, followers of the nonviolent Jesus — imitating his example — practiced total nonviolence.
But after Christianity was legalized and later made the official religion of the Roman Empire, Christians began fighting for the empire. And sadly, Christians have been fighting for empires ever since.
The “just war” theory was developed to offer criteria — like protecting civilians from attack — that had to be met before war could be theoretically morally justified and continued. Most unfortunately, this led to the Catholic Church’s abandonment of total Christ-like nonviolence.