DICKERSON: Help people understand what a conflict with North Korea would be like and how it would be different.

MATTIS: A conflict in North Korea, John, would be probably the worst kind of fighting in most people‘s lifetimes.

Why do I say this? The North Korean regime has hundreds of artillery cannons and rocket launchers within range of one of the most densely populated cities on earth, which is the capital of South Korea.

We are working with the international community to deal with this issue. This regime is a threat to the region, to Japan, to South Korea, and in the event of war, they would bring danger to China and to Russia as well.

But the bottom line is, it would be a catastrophic war if this turns into combat, if we are not able to resolve this situation through diplomatic means.