SEOUL (NYTIMES) - Few people in the world were more surprised and disappointed by the breakdown of the summit between President Donald Trump and North Korea's leader, Mr Kim Jong Un, on Thursday (Feb 28) than President Moon Jae-in of South Korea.

Mr Moon had been so confident of a breakthrough in the meeting in Hanoi, the Vietnamese capital, that he planned to deliver a major speech on Friday (March 1) laying out his bold vision for economic cooperation with North Korea, assuming that a denuclearisation deal between Mr Trump and Mr Kim would lift restrictions on joint inter-Korean economic ventures.

That dream suffered a major setback as Mr Trump and Mr Kim abruptly ended their meeting, unable to bridge their differences.

Mr Kim wanted Mr Trump to remove sanctions, the most potent leverage Washington had to force North Korea to denuclearise. But what he offered the president in exchange was limited to the dismantlement of a single nuclear complex.

Although that complex, Yongbyon, houses facilities that produce plutonium and uranium fuel for nuclear bombs, it is not the only nuclear facility in the North.

The country is widely believed to run at least one clandestine uranium-enrichment plant. The dismantlement of Yongbyon also would not affect the nuclear warheads, long-range missiles and fissile materials the country has already produced.

"It is regrettable that President Trump and Chairman Kim could not reach a complete agreement today," Mr Kim Eui-kyeom, a spokesman for Mr Moon, said on Thursday.

But Mr Moon's government refused to abandon hope, struggling to find a silver lining in the fiasco. Mr Trump said on Thursday that he remained committed to negotiating with North Korea.

"It seems clear that both sides have made more significant progress than ever," Mr Kim, the spokesman, said.

"We believe that through their long hours of deep consultations, the two heads of state both widened and deepened their understanding of what the other side wanted."

Mr Trump called Mr Moon from Air Force One as he flew back to Washington to explain what had happened, and asked him to continue to act as a mediator between him and Mr Kim, Mr Moon's office said.

The collapse of the Hanoi summit was especially painful for Mr Moon. If Mr Kim and Mr Trump return to the personal insults and threats of "fire and fury" and nuclear annihilation they exchanged in 2017, it would be the end of the rapprochement Mr Moon has painstakingly helped to build.

With his approval ratings falling over South Korea's stubborn economic troubles, Mr Moon has hoped to recover some of his popularity by jump-starting his signature policy of helping to advance the North's denuclearisation and improving inter-Korean ties.

That depended on Mr Trump and Mr Kim striking a denuclearisation deal in Hanoi significant enough for Washington and the United Nations to ease sanctions and create room for Mr Moon to push his ambitious plans for economic cooperation with North Korea.

Now, Mr Moon faces criticism that he has been too optimistic and naive, overselling North Korea's willingness to denuclearise.

"South Korean conservatives will intensify their criticism of President Moon's engagement policies for going too far, too fast with North Korea," said Professor Leif-Eric Easley, who specialises in international studies at Ewha Womans University in Seoul.

Mr Hwang Kyo-ahn, head of the main conservative opposition Liberty Korea Party, said on Thursday that Mr Moon's government had been deceiving the people with a "rosy illusion".

Since he took office in 2017, Mr Moon has staked his political fortune on successfully mediating a denuclearisation deal between Mr Kim and Mr Trump. He met with Mr Kim three times last year and helped broker his Hanoi meeting with Mr Trump, as well as the two leaders' first meeting in Singapore last June.

He has tirelessly sold Mr Trump on the merits of negotiating with Mr Kim, saying that the North Korean leader has made a strategic decision to give up his nuclear weapons and focus on rebuilding his country's economy should Washington take corresponding actions, such as easing sanctions.

Mr Moon remained optimistic even after US intelligence chiefs recently warned that North Korea was unlikely to abandon its nuclear arsenal completely because it was considered central to the regime's survival.

As scepticism deepened over negotiating with North Korea, he criticised "forces who seemed to want the era of hostility and strife to persist on the Korean Peninsula".

In recent months, older, more conservative South Koreans have rallied in central Seoul, accusing Mr Moon and Mr Trump of endangering their countries' alliance, particularly over their decision to suspend joint military exercises even when North Korea has not given up a single nuclear warhead.

On Thursday, Mr Trump defended the decision, calling the joint war games too expensive, and said that Mr Kim had promised to stick to his moratorium on nuclear and missile tests.

Analysts have long warned that Mr Moon's betting on cooperation between Mr Kim and Mr Trump leaves South Korea exposed to the whims of the two unpredictable leaders.

"This outcome has been expected, given the impulsive and unpredictable nature of Trump and Kim Jong Un," said Mr Yoo Dong-ryul, a conservative analyst who leads the Korea Institute for Liberal Democracy in Seoul.

Longtime North Korea observers have foreseen intractable stumbling blocks on the road to denuclearisation since Mr Trump and Mr Kim ended their Singapore meeting without sorting out key details, especially regarding what they wanted each other to do to achieve the "complete denuclearisation of the Korean Peninsula".

Washington wanted North Korea to take serious and verifiable "big-bite" steps towards denuclearisation, and feared that lifting sanctions too soon would allow the country to drag its feet and establish itself as a de facto nuclear power.

But Pyongyang's top priority was sanctions relief, considering it proof that Washington was ready for peace and the "new" relations that Mr Trump promised in Singapore.

Analysts had warned against holding a second summit without first sorting out the differences through working-level negotiations.

Mr Trump's abrupt decision must have shocked Mr Kim, who might have seen Trump as desperate for a deal even though the president had repeatedly said he was in "no rush", analysts said.

But Mr Trump's decision comforts people in South Korea and Japan who had feared that he might have settled for only freezing the North's nuclear programme or removing its intercontinental ballistic missiles, the most immediate threat to the Americans, leaving the country with short- and medium-range missiles and their nuclear warheads.

"Trump left the negotiating table, basically telling Kim Jong Un that they will meet again when North Korea is ready to give up its nuclear weapons," said Dr Cheon Seong-whun, an analyst at the Asan Institute for Policy Studies in Seoul. "It must have been a blow to Kim Jong Un."

But analysts also said the collapse of the talks in Hanoi could ultimately lead to a better deal with North Korea.

"As long as both leaders maintain political will and continue talking, we can expect progress in the future," said Mr David Kim, a research analyst at the Stimson Centre in Washington.

"We walked away because we didn't have a deal, but no deal is better than a bad deal."