What threat does North Korea pose to the US?

ABC News' Martha Raddatz reports from Seoul, South Korea on the North Korea nuclear threat.
5:01 | 09/03/17

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Transcript for What threat does North Korea pose to the US?
We are here in Houston, Texas, covering that disastrous flood. You can see the debris behind me. A major domestic crisis. We'll get to all of that. But first, a major international crisis breaking overnight. Just before midnight, U.S. Authorities detected an earthquake in North Korea. An hour later, the government of South Korea said it was caused by an underground nuclear test. 2:35 A.M., north ree Ya made it official. Saying it had tested a hydrogen bomb. A bomb designed to be carried on an inter continental ballistic missile. This is the sixth time north Korea has set off an underground nuclear device and the first such test since Donald Trump took office. According to Japanese authorities, the test bomb was the strong es yet. Four to five times as big as the bomb dropped on nagasaki in 1945. North Korea put out this photo. What they claim to be a nuclear device, small enough to fit on a missile. The great fear all along is that if North Korea can build a nuke that can reach the united States, the president will need to act. The president tweeting the this morning that North Korea has conducted major nuclear test. Their words and actions continue to be very hostile and dangerous to the United States. North Yi Ya is a rogue nation which has become a great threat and embarrassment to China, which is trying to help, but with little success. South Korea is finding, as I have told them, that their talk of appeasement with North Korea will not work. They only understand one thing. For the president, it is a return to the tough tone of early August. North Korea best not make any more threats to the united States. They will be met with fire and fury. Like the world has never seen. Reporter: That was president trump in early August. His tough talk cranking up the heat. If anything, maybe that statement wasn't tough enough. And we're backed by 100% by our military. Reporter: We were with the military in the region weeks ago. I went to san air base in south Korea and witnessed the military flying some of its most advanced war planes. All right, here we go. Reporter: We were given incredibly rare access. A backseat ridealong on an f-16. ? The front seat, captain true Daniels. From the 36th fighter squadron. It's a pretty smooth ride, though, isn't it? Reporter: Just miles away from the demilitarized zone. How far are we from North Korea right now? Right now, ten mile. Reporter: That's as close as you can get? That's as close as we can absolutely go. Reporter: Flyi iing through one of the most dangerous Zones in the world. They fly about 17 sorts each week. It must be strange. You're not in a war zone. But it can happen at any time. I'll tell you, it definitely gives you a real purpose to waking up in the morning. That's our mission here is to be ready to fight at a moment's notice. That's why -- our motto is we're ready to fight tonight. Reporter: Back on the ground, I asked what it would look like if the tensions boiled over and real conflict began? Jt we prepare to deter north Korea and the aggressive stance they take. If ultimately our efforts to deter them fails, we're mobilizing to defend the south Koreans and their way of life and ultimately the brothers and sisters in arms. Reporter: Think people think, oh, the military, they're ready forwar. You're the last guys who want to go to war, right? Oh, sure. No one wants war to erupt on the peninsula. The best way to prevent a war is to be ready. Reporter: Always ready there at san. The threat of war ebbs and flows with every test, every response. Despite his fire and fury warning, president trump sounded a more conciliatory note last week. Saying this about Kim Jong-un. Kim Jong-un, I respect the fact that I believe he is starting to respect us. I respect that fact. Very much. Respect that. And maybe -- probably not -- but maybe, something positive can come about. Reporter: The overnight nuclear test anything but positive.

This transcript has been automatically generated and may not be 100% accurate.

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