President Trump and North Korean leader Kim Jong Un met in Hanoi, Vietnam, on Wednesday — their second meeting since their historic summit in Singapore last year.

But the negotiations came to an abrupt end after Kim demanded that the U.S. lift all sanctions on the rogue regime without first securing its commitment to denuclearization.

“Sometimes you have to walk,” Trump told reporters afterward.

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In a tweet Sunday, Trump emphasized that he expected a "continuation of the progress" made at their first meeting, particularly when it comes to denuclearization. There had even been some speculation that the pair would finally call for an official end to the Korean War, which came to a close in 1953 with an armistice.

Their first summit last June also ended without firm deals regarding the North's nuclear disarmament and triggered a months-long stalemate in negotiations.

The world leaders weren't the only ones involved in the summit this week. Here's a look at some of the officials who played critical roles in the discussions.

Kim Yong Chol

Kim Yong Chol, North Korea's former spy chief and a vice chairman of the ruling Workers' Party, accompanied Kim on his long train ride to Hanoi, the North's official Korean Central News Agency confirmed. Chol has been a key negotiator in talks with the U.S., and Kim Yo Jong, the leader's sister.

He is often referred to as Kim's "right-hand man" and was also present during the Singapore summit.

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Chol, who was infamously banned from traveling to the U.S., according to the BBC, was invited to meet Secretary of State Mike Pompeo in New York City to deliver a letter from Kim a month prior to their first summit. Trump called the meeting with Chol a "great start" at the time.

Mike Pompeo

Secretary of State Mike Pompeo joined Trump in Hanoi and will remain in Vietnam through Feb. 28 before heading to the Philippines to meet President Rodrigo Duterte and Secretary of Foreign Affairs Teodoro Locsin, Jr., according to his public schedule.

On "Fox News Sunday," Pompeo said he was hoping for a "substantive step forward." But, he cautioned, "it may not happen, but I hope that it will."

"President Trump has also said this is going to take time. There may have to be another summit. We may not get everything done this week," Pompeo said.

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Pompeo said he hoped to put a "road map" in place, but would not discuss the possibility of declaring a formal end to the Korean War or pulling some American troops from South Korea, in keeping with his stand against publicly discussing the issue that could arise during the negotiations.

Pompeo said he believes North Korea remains a nuclear threat, though Trump tweeted after the Singapore summit that "there is no longer a Nuclear Threat from North Korea."

Le Hoai Trung

Vietnamese Deputy Minister of Foreign Affairs Le Hoai Trung is tasked with ensuring everything runs smoothly while the world leaders are in Hanoi.

"Security will be at the maximum level," he told reporters at a briefing meant to showcase the nation's efforts to welcome Kim and Trump.

Le said he only learned the second summit would take place in Hanoi "mid-February," giving officials only 10 days to prepare," according to the South China Morning Post.

"[A successful summit] is an important priority for Vietnam’s foreign affairs in 2019," Le told the newspaper, noting that he hopes to "demonstrate our foreign policy as a contributor to peace and our role as a responsible actor of the international community."

Nguyen Manh Hung, the leader of the information ministry, said the 3,000 journalists from 40 countries expected in Hanoi could rely on his agency as "you'd count on a family member."

Kim Yo Jong

Kim's sister Kim Yo Jong, a top decision-making body, was named part of Pyongyang’s delegation to Vietnam, according to Korea JoongAng Daily. During the Singapore summit, Kim Yo Jong spent time with Pompeo.

It's no surprise Kim Yo Jong is heavily involved with the second summit, as she serves as one of Kim's "closest aides," managing her brother's schedule, bodyguards and meetings.

“Kim Yo Jong has a bigger policy portfolio [than her brother], and is more powerful than we’re giving her credit for,” Michael Madden, the director of North Korea Leadership Watch and a visiting scholar at the US-Korea Institute at Johns Hopkins SAIS, previously told Fox News.

Ri Yong-ho

North Korean Foreign Minister Ri Yong-ho already traveled to Hanoi in December to better understand Vietnam's economic reform, according to Quartz.

He was reportedly spotted traveling on the green-and-yellow armored train alongside Kim as they headed to Hanoi.

Fox News' Madeline Farber and the Associated Press contributed to this report.