“North Korea best not make any more threats,” Trump told reporters.

(In response, North Korean state media said the Hermit Kingdom is reviewing plans to strike U.S. military targets in Guam.)

The president's comments came after a report in The Washington Post that North Korea has successfully produced a miniaturized nuclear warhead that could fit inside its ballistic missiles, crossing a key threshold on the path to becoming a full-fledged nuclear power.

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Trump's harsh language against North Korea was interpreted by some foreign policy analysts as a break from the ineffective diplomatic language that has governed Washington-Pyongyang interactions for years.

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One North Korea expert, Robert E. Kelly, called Trump's threats “unnecessary, scary, irresponsible.”

A professor at Pusan National University — who gained viral Internet fame in March when his young daughter crashed his televised interview with BBC — Kelly spoke to The Post about what he meant, and what to expect going forward.

POST: Let’s address Trump’s comments. Your tweet said they were “unnecessary, scary, irresponsible.” How so?

KELLY: “Unnecessary” in the sense that the North Koreans already know that we dislike them, that we want them to act differently and so on and so on. And just a few days before, Secretary Tillerson was on TV saying “we’re not your enemy” and Trump goes off and says this.

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He just undercut his own secretary of state. So that’s what I meant by unnecessary.

“Scary” because what he said sounds like Old Testament-style rhetoric. “Fire and fury.” He’s like some prophet from the Old Testament talking about fire and brimstone.

And “irresponsible” because it sounds like Trump shooting his mouth off again. Maybe his national security team approved of that kind of language, but it sounds a lot like what Trump does on Twitter, which is shooting his mouth off and saying stuff and his national security people have to walk it back in the next couple of days.

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Now the whole world is talking about it. People like you and me have been spending the last five or six hours trying to figure out if Donald Trump is trying to start a nuclear war. And that’s what people are asking me. People are calling me up: Oh this isn’t just a war, Donald Trump wants to use nuclear weapons, fire and fury.

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POST: You’ve said before that “much of the overheated rhetoric coming from Trump administration about North Korea” is actually to pressure China. Who is the audience for Trump’s warnings?

KELLY: My sense is that there are two ways to read these things.

The optimistic one is that Trump got this cleared by his national security staff and he’s sounding a little unhinged or angry because he’s playing the madman role. And the point of this role — not that he’s actually a madman, but to pressure the Chinese into coming around.

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He plays this sort of game and the Chinese are like: “Oh, my God, he might actually start a war and kill us all; let’s go pressure North Korea.”

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This is a way of Trump signaling to China to get serious about North Korea — which, to defend the president, is not necessarily a bad idea because I do think China still has a lot of leverage over North Korea. The best way to resolve the North Korean issue peacefully is to get the Chinese to push the North Koreans harder. I know that’s pretty disputed today. A lot of people just don’t think the Chinese have that weight. But I do.

The negative interpretation is that Trump just shot his mouth off. And now the whole world is like: “Oh, my God, Trump is as unpredictable as Kim Jong Un, and we’re going to have nuclear conflict between these two schoolyard bullies who don’t know how to back down.”

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North Koreans didn’t waste any time at all. One hour after that comment they were talking about nuking Guam.

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This is just bickering. This is all rhetoric. This is not going to happen.

North Koreans are not going to nuke the Americans out of the blue. The North Koreans don’t have offensive intentions. Attacking the United States would be suicidal. The Americans would respond with so much force, North Korea would just be wiped off the map. We know this.

The North Koreans know this. They’re not apocalyptic ideologues like Osama bin Laden, willing to risk everything on some suicide gamble. The North Koreans are pretty rational. They are pretty tactical. They’ve been smart over the years. It would be very out of character for the North Koreans to suddenly launch a weapon at San Francisco. So I don’t buy that at all.

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POST: Trump’s “fire and fury” statement echoes North Korea’s own threats, and some supporters have suggested that nuanced statements in the past have been ineffective and Trump is speaking in a way the North Koreans would understand. What do you make of that?

KELLY: The thing I don’t like about that though is that the United States isn’t some pesky, rogue country with a history of doing crazy stuff and dealing drugs and counterfeiting like North Korea.

North Korea has a reputation as a rogue. We don’t expect it to act any better and it’s a small part of the global economy that’s not really that relevant for global rules.

When the Americans act that way, when the Americans start talking like that, it sends signals to everybody. The center isn’t holding. The Americans are expected to be better than this. We don’t talk this way, in the same way we don’t expect the South Koreans to talk the same was as the North Koreans do. We expect more from democracies, we expect more from liberal countries.

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I think it’s one of the reasons people like you and I are having this conversation, because it’s so uncharacteristic for American leaders to talk like this. Maybe it’s going to work. Honestly I haven’t thought that far. But it’s risky, it’s really risky. Because it sends a signal to everybody else out there that: Hey, you can’t trust the Americans, they might launch a nuclear war.

POST: How do you think Trump’s comments will be received in North Korea? How about in Japan or South Korea?

KELLY: We already know how North Koreans are going to take it. An hour or two later they threatened Guam. That’s how North Koreans always respond to threats. They always reach for the most outlandish rhetoric: Really aggressive, personal insults against the president of South Korea and the United States, the racism and all that.

It doesn’t surprise me at all that the North Koreans immediately went over the top by threatening a nuclear strike on American territory. That’s why we shouldn’t get into these kinds of war of words with the North Koreans. It’s not going to work. It’s not going to lead us anywhere.

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The South Koreans and the Japanese will sort of roll their eyes and say “what is going on over there.” This is just Trump the unhinged. That’s what I’m really worried about — that our allies in Asia are increasingly thinking we are unreliable because the president’s kind of off his rocker.

POST: You tweeted Tuesday, in all-caps, “WE DON'T HAVE TO BOMB NORTH KOREA.” What are America’s options? How likely are China and Russia to stick to sanctions?

KELLY: Sanctions are the most likely, peaceful way to resolve this. Chinese economic pressure through sanctions, enforcement that leads to pressure on the elite bottom line in North Korea, not the popular bottom line. When you get factions in North Korea to start fighting over diminishing resources, that’s the kind of pressure we’re looking for. That can only come around if China plays ball.

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If that doesn’t happen, and that hasn’t happened for 15 years, then my sense is missile defense. But you get a lot of push back on this, too. A lot of tech people say missile defense is a boondoggle, it doesn’t work, THAAD is overrated. My own sense from the briefings I’ve seen over the years about missile defense is that THAAD is at least reasonably effective. It’s a start.

POST: What next?

KELLY: I think the North Koreans will not stop the missile testing program. The Americans will slowly adapt to that in the same way it adapted to the Russia, China and Pakistani nuclear weapons. We’ve learned to adapt and live with those, and we’ll do the same with the North Korean ones as well. We will adapt even if Trump doesn’t admit it.

In the next three or four days, my guess is that the Trump national security staff will go out and clean up the remark and say we didn't exactly mean this. We want to have talks, go to the U.N. etc.

POST: What should people who are paying attention to the North Korea situation for the first time know?

KELLY: I would say two things.

Consider that for 70 years, North Korea has had the opportunity to do major damage to South Korea, later Japan, eventually the United States, both against American forces in the region and now against the American homeland. It’s had opportunities for a long time and has never gone after them.

North Korea now has a long history of restraint, actually. It has a long history of tactical provocation. But North Korea has never gone over the edge. It has always pulled back.

And that leads a lot of us in the analyst community to believe that the North Koreans do not intend to use nuclear weapons. So all of this hysteria, this “World War III is around the corner” kind of stuff, is highly unlikely because the North Koreans have had the opportunity for a while. Look at North Korea’s past behavior as a predictor of future behavior.

The second thing I would say is that if there really is war coming, the big reveal for that would be an evacuation or call for evacuation of Americans living in South Korea. That is the big red flag. So if you see the Americans are told to get on a ship at Busan and go to Japan, you know the American airstrike is coming.