I recently spent several weeks in South Korea.
One tour guide talked about the threat of North Korea. She said that her feeling is, bring it on! South Korea has twice as many people as North Korea. Every man in South Korea has to serve in the military for two years (unless physically incapable), and remains available for military service.
And it is not like the North Koreans have been completely peaceful since the cease-fire. They try things several times a year. The security near the border between North and South is impressive--metal fences topped with coils of concertina wire, guard posts, flood lights, etc. Where freeway overpasses cross a river that might serve as an ingress for North Korean forces, concertina wire is wrapped around the supporting columns to prevent anyone from climbing up. Trump could learn a thing or two about border fences from the South Koreans--he probably already has. He has a hotel there, in Seoul.
Tunnels negate most or all of that. Did he talk about tunnels?