When Moon Jae-in's White House visit was first proposed, the South Korean leader would be coming to brief U.S. President Donald Trump on strategies for his imminent summit — a groundbreaking, deal-making session between the United States and North Korea that could, in Trump's words, be "a very special moment for World Peace".

The highly anticipated meeting between Kim Jong Un and myself will take place in Singapore on June 12th. We will both try to make it a very special moment for World Peace! —@realDonaldTrump

No one is so optimistic anymore that Trump can convince North Korean leader Kim Jong-un to give up his nuclear weapon program.

In fact, as Moon arrives in Washington today the two leaders will almost certainly ask themselves whether Trump should even meet Kim in Singapore on June 12. Could it turn into an embarrassment?

Trump surprised senior U.S. officials when he accepted Kim's invitation for a summit back in March. But the president has reportedly spent the weekend agonizing over the decision. He quizzed his aides on the wisdom of the meeting, then called Moon in Seoul on Saturday night for advice.

Success seems less likely

According to the Washington Post, which first reported the call, the conversation lasted less than half an hour, with Trump asking Moon to explain the North's sudden harsh tone this past week. Pyongyang vowed not to be forced into a corner and threatened to cancel the summit if Trump insisted on "nuclear abandonment" — total disarmament.

It also abruptly pulled out of meeting with the South, complaining about joint military exercises involving U.S. and South Korean forces.

Neither Seoul nor Washington would comment on the details of the weekend conversation. Moon's office would only say "the two leaders will work closely and unwaveringly for the successful hosting of the North Korea-U.S. summit."

Success seems a lot less likely than it did after a chummy meeting between Moon and Kim on their shared border last month and the release of three American detainees by Pyongyang as a goodwill gesture three weeks ago.

Aiming for U.S. concessions

U.S. and South Korean officials are also closely watching to see if the North goes ahead with a pledge to destroy its Punggye-ri nuclear test site near the Chinese border this week. It has promised to invite journalists from the U.S. and several other countries to witness the demolition, and satellite images have shown what appear to be viewing stands being built nearby.

Observers in South Korea say no one should be surprised by all the conflicting signals.

In this May 9, 2018, file photo provided by the North Korean government, U.S. Secretary of State Mike Pompeo, left, shakes hands with North Korean leader Kim Jong-un during a meeting in Pyongyang, North Korea. Over two such meetings, Pompeo was unable to get much clarity on Kim's intentions. (Korean Central News Agency/Associated Press)

"This is the typical brain game of North Korea," said Cheon Seong-whun, a former adviser to the South Korean government and now a research fellow at Seoul's Asan Institute. He's also a graduate of the University of Waterloo.

"It's a well-thought, long-planned, deliberate game plan of North Korea, aimed so that it remains a nuclear weapon power," said Cheon. He said the goal is to force Washington to make as many concessions as possible before the summit.

Differing views of 'denuclearization'

In particular, experts in Seoul say Kim's aim is to trade a modest reduction in the North's nuclear arsenal in exchange for promises from the U.S. to keep any nuclear-capable ships, submarines and planes away from the peninsula, as well as a long-term pledge to pull its troops. About 28,000 American forces are based in South Korea.

Cheon said this is North Korea's idea of denuclearization, while the United States has been using the term to mean total nuclear disarmament, or as U.S. Secretary of State Mike Pompeo puts it, "complete, verifiable, irreversible denuclearization."

That's a goal also set by previous U.S. administrations that proved impossible to achieve and one that few observers in the South believe Pyongyang would consider now that its program is virtually complete.

Trump has suggested he can achieve this goal in negotiations with Kim. But even some of his advisers worry he would settle for much less just for the sake of a deal he can flaunt, perhaps with the goal of a Nobel Peace Prize that, as he puts it, "everyone thinks" he deserves.

Riding the tiger

South Korean observers like Shin Chang-hoon also worry that Moon will push Trump to go ahead with a "ridiculous" summit because he has staked his career on inter-Korean peace.

"Moon Jae-in has been playing the role of matchmaker and he has heightened the expectations of both sides," said Shin, a nuclear non-proliferation expert and senior researcher at the Korea Institute for Maritime Strategy.

"Now he's riding a tiger that he cannot control," he said.

Another wild card in all of this is the role of China.

China, North Korea relationship warms

Since March, Kim has also been warming up to Beijing. The two countries are traditional allies, but their relationship was strained over North Korea's nuclear and missile tests and China's enforcement of UN sanctions that have choked the North's economy, especially fuel. Those sanctions continue for now, though Washington is worried China has relaxed enforcement along the frontier between the two countries.

In a tweet yesterday, Trump complained that "the border has become much more porous" and said, "China must continue to be strong & tight on the Border of North Korea until a deal is made."

China must continue to be strong & tight on the Border of North Korea until a deal is made. The word is that recently the Border has become much more porous and more has been filtering in. I want this to happen, and North Korea to be VERY successful, but only after signing! —@realDonaldTrump

Indeed, the relationship between Beijing and Pyongyang seems much more amicable. Chinese President Xi Jinping has hosted Kim at two summits in the past weeks, one of these involving a stroll along a beach popular for Chinese newlyweds' wedding photos.

North Korea and South Korea have been split for 65 years, but the divide continues to fuel tensions today 5:01

Little has been revealed about the discussions, but Chinese experts who are usually plugged into the government's official positions say it's in China's interest to encourage the U.S.-North Korean summit by keeping up the economic pressure.

Besides, "if the violations are caught, China could lose credibility, reputation and face," said Cheng Xiaohe, a prominent analyst at the School of International Relations at Beijing's Renmin University.

"China would be stupid to violate United Nations resolutions just to support North Korea," he said.

Beijing may share Pyongyang's goal of reducing the American military presence on the Korean Peninsula, but it hasn't been happy with Kim's nuclear ambitions. Among other things, it's afraid a failure in negotiations will "provoke more serious confrontations than before," said Cheng, including threatened military action by the U.S., right on China's doorstep.