Simple, you establish a line between you and the invading hordes, a line of infantry armed with machine guns backed up with mortors and artillary. Establish a no mans zone we now call a free fire zone and conduct both random artillary barrages and sortees by various strike aircraft. In front of the infantry ensure that area is mined. Sounds pretty impregnable to me. How many “mass migrations” have there been from North to South Korea since 1954? I bet NONE.
cobalt 60 ...