In seeking a biblical understanding of the theory and practice of war, it is important to take note of everything the Scripture has to say about the topic – no matter how inconsequential it may seem. Thus, when we look to the New Testament we eventually find ourselves in Luke 3:14. There we read about a group of soldiers who came to John the Baptist and asked;
And we, what shall we do?” And he said to them, “Do not extort money from anyone by threats or by false accusation, and be content with your wages.
It is improbable these were non-Jewish soldiers. Most likely they were Jewish men or proselytes in the employ of Herod Antipas or Philip. They may have been Jews who served in the Roman Legions (this was common enough in the first century). The third possibility is that they were temple guards employed by the Priestly caste. In any case they were men who were armed, and who had authority beyond their own bluster or physical presence.
In a nut shell what John said is that they must not use their military might as a means of personal gain. He tells them they should not “shake down” people for money or other personal favors. The Greek literally means “to shake thoroughly” as in, “to do violence.” It is easy to see how this would be a temptation even if the soldiers were of Jewish blood. If they found they were a little short of cash, who would be able to stop them from “shaking down” an innocent bystander? After all, the Jewish people were under the heel of the Romans. A man who cast his lot in with the Empire was largely protected from any consequences he might face for grinding that Roman heel just a bit harder.
John also told the soldiers not to accuse anyone falsely. To better understand John’s prohibition, we need to remember the various military entities also functioned as local police. John was saying they should not use their authority to blackmail anyone. If the attempt to physically shake someone down was unsuccessful, well, it would have been an easy matter to cook up a story against a local peasant and falsely accuse him of a crime. After all, how could a private individual hope to prove his case against the police?
John finally admonished the soldiers to be satisfied with their wages. The wage of the Roman Legionnaire (or provincial soldier), would have consisted of money and rations and presumably barracks housing (at least while on duty). Thus we see that John summed up the first two demands by telling the soldiers to live on the salary they received.
Now then, how does this add to our understanding of a biblical theory and practice of war? Well, if no individual soldier is supposed to shake someone down for money or accuse them falsely for gain (but to be content with what they already have), it stands to reason that no national army should threaten war or actually go to war in order to extract wealth from another nation. (Take note: John did not qualify his demands by saying it was okay to do shake down or extort someone if a commanding officer ordered it.)
Therefore a desire for territory, natural resources, increased industrial capability, a greater population base and so on, are not legitimate reasons for war.