Why send troops to cut off an area when you can have a 
  24/7 patrol of small, cheap drones that'll 
  kill anything larger than a rabbit ...?

That sounds like an automated smart minefield, which can be mobile. I'm sure we will see that soon, because the usefulness of such things (if they work) will be too hard to pass up.

I hope that Philip K. Dick's Second Variety [1] is not the way that ends up, though.

1: http://en.wikipedia.org/wiki/Second_Variety