SEOUL, South Korea — Few people in the world were more surprised and disappointed by the breakdown of the summit meeting between President Trump and North Korea’s leader, Kim Jong-un, on Thursday 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 laying out his bold vision for economic cooperation with North Korea, assuming that a denuclearization 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 denuclearize. But what he offered the president in exchange was limited to the dismantlement of a single nuclear complex.