HARI SREENIVASAN, PBS NEWSHOUR WEEKEND ANCHOR: Defense Secretary James Mattis said today that a reduction in U.S. troops participating in joint military exercises with South Korea this week is not related to increased tensions with North Korea. More than 17,000 U.S. troops are expected to be a part of the Ulchi Freedom Guardian Drills that start Monday and last for 10 days. Twenty-five thousand participated last year. North Korea called the bi-annual exercises, quote, reckless behavior driving the situation into the uncontrollable phase of a nuclear war.

For some perspective, I'm joined from Denver, Colorado, by Ambassador Christopher Hill, the chief U.S. negotiator with North Korea from 2005 until 2009, who has also served as a U.S. ambassador to South Korea.

Unrelated that we are decreasing the number of troops?

CHRISTOPHER HILL, FORMER U.S. AMBASSADOR TO SOUTH KOREA: Yes, I think it is unrelated. I mean, over the years, we've had more troops and fewer troops. It kind of depends on what they are exercising, what sort of maneuvers are they doing and what sort of units are coming often from the U.S. mainland to participate.

So, I think what is also pretty clear is these are exercises aimed at protecting South Korea and they're exercises pursuant to the U.S. commitment in the U.S. Republic of Korea alliance where we will come to South Korea's protection if they are attacked.

SREENIVASAN: You know, the North perceives this as drills that are staging area of an invasion of the North. I mean, it's — the perception correct or not, but this is clearly for them a source of some consternation every time they happen.

HILL: Or maybe it isn't, because yes, they have complained about these exercises ever since they first started, but they also know because they have pretty smart people looking at these things that these are defensively oriented exercises, exercises designed to have U.S. military work closely with our counterparts in ROK counterparts in the event of a North Korean invasion.

SREENIVASAN: Give us a sense of the readiness that exists along the peninsula between Japan, the Koreas and elsewhere. Obviously, it was ratcheted up, the rhetoric between both sides in the past couple of weeks, and the firing of the test missiles in the first place. How ready and prepared are all of these countries to stop a missile that might take off from North Korea from hitting anyplace that we care about?

HILL: Well, antiballistic missile technology is not as advanced as ballistic missile technology. So, the hope is that we can hit these incoming missiles with kind of latest generation anti-ballistic technology, including the so-called THAAD System, the Terminal High Altitude Area Defense System. I'm not among those people who believe that we're about to, you know, be involved in a missile war with North Korea, let alone a nuclear war. But we need to continue to upgrade these systems.

SREENIVASAN: You know, as we do these military drills, how realistic is sort of a military option against the North considering how many people live in Seoul just 35 miles from the border?

HILL: To launch a so-called preemptive strike against North Korean nuclear facilities, first of all, it's hard to say we would have the capability really of getting all these nuclear facilities. You know, North Korea is a pretty riveted country, pretty dug in country. I mean, they've got all kinds of tunnels and places underground to store things. So, I think, one, that would be pretty difficult.

And secondly, there are some 20 million South Koreans within in range of some 14,000 North Korean artillery tubes and while we have tremendous anti-battery capability, I mean, I would not want to be a North Korean firing an artillery shell at the South because within seconds, we would eliminate that installation. But nonetheless, they would be able to get a number of these shots off and we would have rather substantial civilian casualties. And frankly we'd be into a second Korean War.

SREENIVASAN: All right. Ambassador Christopher Hill, thanks so much.

HILL: Thank you.