October,
Instead they backed away, and allowed the French to fall to pieces all by themselves through indiscipline and lack of forethought.
The enemy I am thinking of is neither undisciplined nor lacking in forethought.
And presumably Paul knew what a backplate was, but significantly never mentioned one.
I believe in strategic retreat in the physical and the emotional realm.
I do not believe in retreat in the spiritual realm, at least when we're wearing our armor. 
mudpup
Where we probably differ is in the composition of that 'enemy', Mud. (Although I doubt we would fall out over it.) To my mind there is enough evil around in people, and no need to project it onto any kind of demon. However, each is entitled to their own view, of course.
I personally believe in strategic retreat in any situation where the person standing their ground is going to get hurt more by staying than by leaving. Then the angels can do the standing, while we do what we do best; pray.
Paul didn't need to mention the backplate. He was talking to people who had seen Roman armour. The two are connected; you cannot have one without the other. They fasten together with straps over the shoulder and at the sides. Its like today saying we are going for a drive in the car. No need to mention fastening seat belts; it is assumed and taken for granted. You only need to mention it if there is a chance that you could drive without wearing seat belts. Under UK law you can't legally, so there is no need to mention it. Same here; if you can't put on a breastplate without a backplate, and the leather straps, therefore there is no need to mention the straps or the backplate.
A similar parallel might be the flackjacket. If a person says they are putting on a flackjacket they do not describe all the components, or what it covers. The name includes all the component parts, for anyone who has seen a flackjacket. For anyone who has not, there would remain ambiguity about which parts are covered and which are not.
I know the 'front only' image is a current one, and I have heard it before, but I am afraid it is a modern allegorical image deriving from lack of knowledge of ancient armour, and it does not fit historical reality, and would have therefore had no resonance with Paul or his listeners, imo. I am sorry. The only reason I even mention it is because I think it presents a distorted picture of how we are to approach evil, in whatever form.
