Kim Jong Un's historic call for peace also included an unspoken message to U.S. President Donald Trump: North Korea won't surrender its nuclear weapons easily.

The agreement Kim reached Friday with South Korean leader Moon Jae-in declared "a new era of peace" and sought a formal end to the seven-decade-old Korean War. While it said both countries committed to a nuclear-free Korean Peninsula, it gave no details on concrete steps to achieve it.

More ominously, North Korean's state-run media released a commentary shortly after the agreement was announced calling on the U.S. to drop its "anachronistic hostile policy" and "bad manners." It declared North Korea a "world-level politico-ideological and military power" and said it would contribute to building "a world without nuclear weapons."

"If North Korea gets a deal with the United States that serves their interests, they'd be happy with that," said Mintaro Oba, a former U.S. State Department official who worked on North Korean issues. "But in the event that things fall apart, they want to be able to blame the United States and alienate it from its South Korean ally in the process."

Trump expressed optimism on Friday -- even declaring "KOREAN WAR TO END!" -- while simultaneously sounding a note of caution. "Good things are happening, but only time will tell!" he said on Twitter.

Friday was filled with unprecedented scenes, starting with Kim's step over the ankle-high concrete slab dividing the Korean Peninsula -- and then his walk back across the border hand-in-hand with Moon. Later, the leaders planted a tree and talked privately for 30 minutes in front of television cameras.

Kim called for frequent meetings between the leaders, which would be a major shift given only three summits have taken place since the war. And he capped it off with live remarks to reporters, something no other North Korean leader had done before.

"We will continue to make efforts so that this agreement doesn't repeat the bad history of the other North-South agreement," Kim said.

Now the attention turns to Kim's meeting with Trump. No date or place has yet been set for what would be the first ever summit between leaders of North Korea and the U.S., although Trump has said it will be held by early June.

Trump's optimism stems in part from Secretary of State Mike Pompeo's secret trip to North Korea over Easter weekend to meet with Kim. Earlier this month, the president said the visit "went very smoothly and a good relationship was formed" while hailing the merits of denuclearization.

The commitment to ‘complete denuclearization' is ambiguous.

Still, it remains to be seen whether the agreement will lead to lasting change. Past negotiations have fallen apart over inspections, weapons tests and disputes over economic aid.

It's also unclear what each side means by the word "denuclearization." Kim may insist that the U.S. removes its nuclear assets that defend allies South Korea and Japan before he gives up his weapons.

"The commitment to ‘complete denuclearization' is ambiguous, and subject to different interpretations," said Youngshik Bong, a researcher at Yonsei University's Institute for North Korean Studies in Seoul. "It can be interpreted as North Korea getting rid of all warheads, or North Korean demands on the U.S. military in South Korea."

So far, it's not apparent if any of those sensitive topics have been discussed. Trump has vowed to maintain sanctions on North Korea until he gives up his nuclear weapons, raising questions over the sequencing of any disarmament.

The next few months are crucial. For all the warm words, Kim still claims to possess the capability to strike the U.S. with a nuclear weapon -- and Trump has threatened war to stop him.

"Considering the level of North Korea's nuclear and missile capabilities now, the declaration today falls short of expectations," said Shin Beomchul, a director at the Asan Institute for Policy Studies who studies North Korea's military. "The two leaders left most of the details for denuclearization on Trump."

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