In the movie “World War Z”, the North Koreans were, indeed, the only nation to survive. BUT, they did it by pulling the teeth of all of the 23 million population.
Seriously, though. You don’t need a standing army to shoot your ravaging neighbor or mother-in-law. American citizens are among the most heavily armed on the planet.
IIRC the book has a far more horrifying fate for North Korea. They lock up the bulk of the population into caves in the mountains to wait it out. Leaves it up in the air whether that was a good decision, or bad decision.