The best way to deter an invasion is to arm the population "Behind ever blade of grass there will be a rifleman' to mis quote a japanese general while discussing why it would be a BAD idea to invade the US.

That holds true to this day. Everyone should have a rifle, a shot gun, a hand gun. Perhaps a couple of rocket launchers (stinger and javelin) There's no good reason for the civilian population to NOT be armed as well as the military.