King Offa's Border Policy

I was reading about the border policies of King Offa, the Anglo-Saxon king of Mercia. He made no attempt to conquer Wales. Instead, he built Offa's Dyke, a boundary-marking ridge, and every few years he raided Wales. (Click here to read more.)

That is a common imperial boundary policy. Mark the boundary, so the barbarians know how far they're permitted to loot to, and if the barbarians make trouble, launch a punitive expedition and burn down some of their houses. Don't attempt to conquer them, because it's not worth the expense of keeping a garrison in a wasteland that won't yield much taxes anyway.

That's what we should have done in Iraq and Afghanistan, with the addition of pre-emptive expeditions to stamp out any preparations for barbarian raids.