“The United States has great strength and patience, but if it is forced to defend itself or its allies, we will have no choice but to totally destroy North Korea,” Trump said, before using a belittling nickname for North Korean leader Kim Jong Un. “Rocket Man is on a suicide mission for himself and for his regime.”

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But on Tuesday, when Trump returned to the General Assembly, he adopted a much different tone. “I would like to thank Chairman Kim for his courage and the steps he has taken, though much work remains to be done,” Trump said.

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So what is responsible for this remarkable turnaround? There are any number of plausible answers, but much of the credit should go to South Korean President Moon Jae-in, who jump-started inter-Korean diplomacy this year and has held two face-to-face meetings with Kim — including one in North Korea.

With Kim a no-show at the United Nations this week — there were widespread rumors that a second Trump-Kim summit could take place in New York — Moon is very much the man of the moment for Korea watchers.

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Trump went out of his way to thank the South Korean leader during his speech, notably mentioning his name before Japan’s Shinzo Abe and China’s Xi Jinping. Later on Tuesday, at a Council of Foreign Relations event, Moon was greeted by a rapt audience of some of the world’s foremost foreign-policy experts.

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“You are a welcomed guest here in New York and here in America,” Kevin Rudd, the former Australian prime minister and current president of the Asia Society, said as he introduced Moon. “This has been an extraordinary year in inter-Korean relations and the principal dynamic in that, Mr. President, has been yourself. You’ve taken risks.”

During a short speech and question session with the audience, Moon reiterated that he thinks Kim is sincere about giving up his nuclear weapons. He also emphasized that there needs to be a second U.S.-North Korea summit where a formal end to the Korean War could be discussed as a possible first step to a permanent peace treaty.

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Despite the warm welcome, there was undoubtedly plenty of skepticism among those in the room. Many experts in Washington, and not just Korea hawks, say North Korea will never give up its nuclear weapons. Using the end of the war as a first step, they say, will only weaken the U.S.-South Korea alliance instead of warming relations with Pyongyang.

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Even members of the Trump administration, which is formally committed to denuclearization, have expressed real skepticism. The latest to do so was CIA Director Gina Haspel: “The regime has spent decades building their nuclear weapons program,” she told reporters Monday. “The North Koreans view their capability as leverage, and I don’t think that they want to give it up easily.”

But Moon knows that, for all intents and purposes, he has an audience of one: President Trump.

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At the Council of Foreign Relations event, he repeatedly brushed aside negative points and praised Trump for making a “very brave decision” by engaging with Kim directly. The left-wing South Korean president will do a major interview with Trump’s favorite news channel, the right-leaning Fox News, where he probably will repeat that upbeat and obsequious message.

Moon has not only helped turn around North Korea’s relations with the United States — he has also turned around his own country’s relationship with Washington.

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On Monday, Trump met Moon for the long-awaited signing of a revised free-trade agreement between the two nations. Last year, attempts to revise the agreement — negotiated by the George W. Bush administration and known as KORUS — were a real sticking point between Washington and Seoul. Trump had surprised many South Koreans with his demand to scrap the “horrible” deal.

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“We don’t quite understand what is the main purpose of Mr. Trump attacking Korea by terminating ­Korea-U.S. FTA at this critical time,” said Choi Seok-young, one of the original negotiators of the deal.

The negotiations over amending KORUS took far longer than many had expected — indeed, some trade experts were underwhelmed by the meager results of the talks. “The new KORUS is just the old KORUS with minor tweaks,” tweeted Simon Lester of the Cato Institute.

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But Trump, true to form, excitedly portrayed the changes as a “new trade agreement.” In his speech to the U.N. General Assembly, he highlighted KORUS as an example of how his administration was “systematically renegotiating broken and bad trade deals.”

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Moon understands that Trump needs victories. As The Post’s David Nakamura and Anne Gearan note, Trump did plenty of complaining and threatening in his U.N. speech, offering “sharp warning to the leaders [of] Iran, Syria, Venezuela and China over what he described as their rogue behavior.” When he mentioned North Korea, however, he described it as a qualified success.

Right now, this looks like a winning strategy for Moon — and it is far better than the grim situation on the Korean Peninsula last year. But we may have to check in again next year at Trump’s speech to see whether this new attitude sticks. As we now know, a lot can change in 12 months.