Are you kidding? South Korea would immediately go on the offensive once hostilities broke out. Ask the Russians how well mass infantry attacks stop a determined mechanized invader.
A people who have been starved for years and years, repressed and exploited shamelessly like the North Koreans, make for really lousy soldiers.
That’s why I believe a free people, well armed and no doubt really pissed off at an agressor nation they’ve repeatedly attempted to help, would pulverize any invading force. And right now the South is prepared for the worst.