'Fight Tonight': Rare look at US defense against North Korea

ABC News' Martha Raddatz visits United States' Osan Air Base in South Korea, located just 48 miles from North Korea.
6:09 | 04/16/17

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Transcript for 'Fight Tonight': Rare look at US defense against North Korea
Hello, from Seoul, south Korea. It's evening here in the city of 10 million people. On high alert in a week end of escalating tensions. Hours ago, yet another north Korean missile launch. The fifth of 2017. This time, it blew up seconds after leaving the launch pad. U.S. Officials believe it was likely a medium-range ballistic missile, the kind we have seen before. Still, the latest launch rattled nerves, especially after the Sunday appearance Saturday of what could be new long-range weapons at North Korea's big anniversary parade. Canisters that appear large enough to house a missile capable of hitting the united States. You can see them there rolling past north Korean leader Kim Jong-un. Green camo on huge transporters. If he, indeed, has a missile big enough to reach the united States, and if he can make a war head small enough to fit on it, will the U.S. Be forced the respond. This hour, all the angles on the most important sportstory if tn the world right now. Are the new missiles we saw at Saturday's parade real? Does Donald Trump have a firm red line? Is there room for negotiation or are trump and Kim on a collision course? Vice president Mike pence arrived here in Seoul today, the start of a ten-day Asia tour. With China warning both sides to cool it, North Korea is at the top of the agenda. In a moment, we'll talk to trump security adviser H.R. Mcmaster. He's in Afghanistan, where the United States just proved what it can do with a mother of all bomb strikes on ISIS, a dramatic show of American fire power. We begin with North Korea's warning that it will annihilate military bases here in south Korea. They said in minutes. If the U.S. Tries to take out its nuclear program. We visited the most important of those front line bases, san air base, just south of Seoul. 48 miles from the border with North Korea. We got exclusive and unprecedented access. ??? if North Korea pulls the trigger this could be target number one. Osan air base, just 48 miles from North Korea, well within range of Kim Jong-un's existing arsenal of missiles carrying god knows what. So a hardened bunker. This is serious stuff. Yes, ma'am. This is your third set of blast doors here. Reporter: The first line of defense is always on guard. 4/7. M-16s on constant patrol. Anti-missile batteries primed and ready. Behind these concrete walls and steel doors, the top secret and rarely seen operations center, on high alert, watching for any north Korean missile activity. We live with it every day. So -- the nice thing is for my folks, the mission is staring them in the face. Reporter: While the world worries that North Korea will get a missile that can reach the United States, there's though doubt here at san the threat is up close, dangerous, and continuous. The slogan here, fight tonight. And you hear it often and always with a sense of pride. There are not many places that go into where it says fight tonight in giant letters. It's a little frightening. Absolutely. But, for us, it's our day-to-day training. We're always training at the level to be able to fight at night because we don't know when that call is going to come. We visited the top secret facility. If a missile is aimed at south Korea, the job here, blow it out of the sky. North Korea has conducted just five underground nuclear tests in the last ten years. But missile launches are far more common. Several this year alone. Every one closely monitored on these screens including last night's failed launch. What's it like if here when a ballistic missile is launched? You would think it would be hectic. Due to the level of training of the soldiers here, it's a calm exercise. I would compare to it maybe an air traffic roller tower. Reporter: Right next door, rows and rows of bunk beds. It allows us to do continuous operations for an exteped period of time. Reporter: You have food, everything you need? Food, showers, latrines. Reporter: Thomas begson is deputy xhapder of U.S. And all U.N. Forces in South Korea. What concerns you most? Our biggest concern is he's going to miscalculate. That's always a concern. We want to make it crystal clear to the north Korean leadership that this would be a completely futile endeavor. Were her to challenge this strong alliance. Reporter: Because we have a new president and he, no doubt, Kim Jong-un wants to test him in the same way that that dynamic works, is this, from your time here, from watching here, one of the more dangerous times? I would say that -- there's certainly, you can feel the tensions. Very clearly, this is serious. They need to know that we're prepared. And this defensive alliance is strong. And ready to fight tonight. Reporter: Part of that defense, patriot missile batteries. You'll notice here, in Korea, we keep our launches loaded with live interserpts at all times. So that we're ready to fight tonight. Reporter: Tell me about the batteries just in term of what day can stop, how they can stop them, how that works? Yes, ma'am. This weapons system is capable of defeating a wide range of enemy capabilities from unmanned aerial systems to short-range and long-range ballistic missiles. This battery is designed to search for, detect, and engage those inbound ballistic missiles. Reporter: At san, they're confident in the system, claiming the success rate of the patriot is nearly 100%. And it's a bullet on bullet scenario. The patriot miss T hits the threat missile. And destroyed it. Reporter: A few miles from the patriots, out on the flight line, are the growlers. They have jamming capabilities. Electronic jamming kablts. They deny the enemy the capability on launch surface to air missile. Reporter: Major SHAWN Walsh, an f-16 pilot. Up in the skies at least three times a week. You see the stark contrast. The lights start to come on, you fly along the dmz. One window, it's bright as day with the city of se orvegs ul. The other side of the cockpit, you look out, it's darkness. Reporter: Make no mistake, it's the darkness that causes concern. The lack of hard intelligence. The unpredictability. And the war of words right now have created a region on edge. Kim Jong-un is -- basically saying, that he could strike and take out any of the American military bases. Could that happen? No. No, they could try. But as you saw today out here. You have air defense artillery brigade. The technology, the capability to intercept those kinds of missile ps. They could try. It would be futile.

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

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