Actually there are no minefields in the DMZ, atleast there aren't supposed to be, I assume some could be remnants, but planting minefields in the DMZ would be violating the cease fire as I understand it. And our troops can not guard our borders because of Posse Comitatus. In effect, our troops can not be used to enforce US laws in the country unless specifically authorized. Other then authorizing it(which may not be the best idea) all we could do is make the entire border a military base, or declare war on Mexico and Canada. Of course you're an isolationist so I'm not sure why you even think we need an an army.