Or better yet, what rules would you come up with that make better instructions to these armies if you believe in rules of war?
Well, obviously, you go by the rules elsewhere in the Bible which call on you to exterminate the men, women, boy children, girl children, babies, cows, goats, sheep, and goldfish! Now
that's some real rules of war! Deuteronomy 13:15, for one of many examples.
"The Christian church, in its attitude toward science, shows the mind of a more or less enlightened man of the Thirteenth Century. It no longer believes that the earth is flat, but it is still convinced that prayer can cure after medicine fails." H L Mencken