Open this photo in gallery Chinese President Xi Jinping arrives for an event commemorating the 200th birth anniversary of Karl Marx at the Great Hall of the People, in Beijing, China on May 4, 2018. JASON LEE/Reuters

Zhiqun Zhu is professor of political science and international relations at Bucknell University in Pennsylvania.

Developments around the North Korean nuclear issue since the beginning of 2018 have been so rapid and dramatic that most observers feel as if they are on a roller-coaster ride. Expectations remain high for the historic meeting between Donald Trump and Kim Jong-un despite recent doubts. With North Korea threatening to pull out of the Singapore summit, both South Korea and China appealed to Washington and Pyongyang to stay on course. Amid the uncertainty there were several significant developments.

First of all, the shortest line between Washington and Pyongyang may run through Beijing. Twice, in late March and early May, Mr. Kim travelled to China to meet with President Xi Jinping in an effort to seek China’s support ahead of his meeting with Mr. Trump. TV footage showing a smiling Mr. Xi walking alongside a relaxed Mr. Kim sent a powerful message of a united front. This is a U-turn in the tense relationship between the two leaders, and between the two countries, since they took office in 2012 and 2011, respectively. Notably, North Korea appeared to have hardened its stance after the second Xi-Kim meeting.

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When Mr. Kim first announced his plans to meet with South Korean president Moon Jae-in and Mr. Trump, some suggested that China might be sidelined in this new round of diplomacy. Indeed, it would be a public breakup of the China-North Korea relationship if Mr. Kim went ahead and met with Mr. Moon and Mr. Trump before meeting with Mr. Xi. As it turned out, Mr. Kim went to China before his scheduled meeting in April with Mr. Moon. From then on, it has become clear that China remains an indispensable player on the Korean Peninsula. It is no coincidence that Mr. Trump got on the phone with Mr. Xi shortly after both Xi-Kim meetings. It is more than just a courtesy when Mr. Trump calls Mr. Xi his “friend” and praises him for helping with the nuclear issue.

Secondly, despite North Korea’s dismal record in keeping its denuclearization promises in the past, this time around Mr. Kim seems serious. He already has frozen missile and nuclear tests, and dismantled a nuclear test site. He also released three Korean-Americans held by the North. Such gestures are symbolic and were not possible just a few months ago. Mr. Kim has taken important initial steps and is waiting for reciprocity from Mr. Trump.

Some in the U.S. administration think that by agreeing to meet with Mr. Kim, Mr. Trump has made a huge concession, since a meeting with a U.S. president has long been “craved” by North Korean leaders. Such a condescending view does not augur well for U.S.-North Korean relations. Mr. Kim’s position is secure at home, and the closer ties between him and Mr. Xi will only consolidate his power. A meeting with Mr. Trump will not add much to what he already enjoys. Indeed, Mr. Trump wants the meeting as much as Mr. Kim does since it fits his personality of talking and doing big things, and a successful meeting will also help his Republican Party in the U.S. mid-term elections later this year.

Thirdly, Mr. Xi’s remark that China and North Korea are socialist countries and their relationship has major strategic significance indicates that he intends to revitalize the special friendship between the two traditional allies, though it may not be a return to the golden days when the relationship was as close as “lips and teeth.”

China and North Korea do need each other now. Mr. Kim needs China’s backing and reassurance as he enters talks with the unpredictable Mr. Trump, and China will definitely not abandon North Korea in the complex strategic rivalry in Northeast Asia, especially when the Trump administration has flirted with playing the trade and Taiwan cards. ​

Mr. Xi’s support for North Korea’s shift to economic development suggests that China will likely ease sanctions against North Korea soon, despite Mr. Trump’s claim that he and Mr. Xi agreed to keep the sanctions on until North Korea denuclearizes.

Finally, is Mr. Trump ready for a serious deal that will require change in his approach toward North Korea? ​​If Mr. Trump could just walk away from the Paris Agreement on Climate Change and the Iran nuclear deal, one wonders whether he will keep any agreement with Mr. Kim they reach. The Trump administration should know that while it is demanding a “permanent, verifiable, and irreversible” destruction of North Korea’s nuclear programs, its own credibility is in question.

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Mr. Kim’s promise to denuclearize comes with preconditions. In his view, if “relevant parties” abolish their hostile policies and remove security threats against North Korea, there is no need for North Korea to be a nuclear state. Will the United States ditch those “hostile policies” such as joint military exercises, continued sanctions, reluctance to sign a peace treaty to end the Korean War, etc.?

North Korea stated that it would not be interested in a meeting with one-sided demand for denuclearization. Is Mr. Trump willing to meet Mr. Kim half way and reciprocate Mr. Kim’s initial goodwill with tangible incentives? There have been murmurs about Mr. Trump being a strong contender for the 2018 Nobel Peace Prize. Can Mr. Trump really bring peace to the Korean Peninsula?