By Choe Sang-Hun, New York Times–

When the armored train carrying Kim Jong-un back from his summit meeting with President Trump in Vietnam reached Pyongyang Station at 3:08 a.m. Tuesday, throngs of flower-waving North Koreans greeted their leader with “boundless emotions and excitement,” the country’s state-run media said.

But Mr. Kim returned home empty-handed — without relief from international sanctions — prompting the question of what he will do next: Particularly, will he resume his nuclear and missile brinkmanship to reassert his leverage?

The revelation on Wednesday that North Korea had started rebuilding the partly dismantled facilities at Tongchang-ri, where the country tests technologies for its intercontinental ballistic missiles, raised the specter that Mr. Kim was returning to his provocative behavior.

But experts on North Korea say Mr. Kim may be boxed in: He returned home without sanctions relief amid strong signs that the North Korean economy is continuing to contract. The deepening economic trouble may force the country to return to the negotiating table.

In restarting operations at its missile technology site, North Korea is seeking “to increase its leverage before the next round of talks,” said Koh Yu-hwan, a professor of North Korean studies at Dongguk University in Seoul.

“I don’t think the North will resume missile tests anytime soon and risk the resumption of United States-South Korea joint military exercises and even the talk of a military option by the Americans,” he said.

Mr. Trump expressed his displeasure at the prospect of new testing. “I would be very disappointed if that were happening,” the president said at the White House on Wednesday when asked about reports on the North Korean missile facilities. “It’s a very early report, and we’re the ones that put it out. But I would be very, very disappointed in Chairman Kim. And I don’t think I will be, but we’ll see what happens. We’ll take a look. It’ll ultimately get solved.”

The two-day Hanoi summit meeting ended last week without any agreement on the terms of denuclearizing North Korea, dashing Mr. Kim’s hopes that he could persuade Washington to lift the most punishing sanctions against his country in return for a partial dismantlement of its nuclear program.

Officially, both sides remain committed to dialogue, with Secretary of State Mike Pompeo expressing hope that the United States will send a delegation to North Korea “in the next couple weeks.” And this week, Washington canceled two major joint military exercises with South Korea to keep the diplomatic momentum alive.

But in a telling sign of disappointment, the official Korean Central News Agency’s report on Mr. Kim’s return home made no mention of “comprehensive and epoch-making results” that it had earlier said Mr. Kim was discussing with Mr. Trump in Hanoi.

“The Hanoi summit was a knockout defeat for Kim Jong-un,” said Lee Seong-hyon, an analyst at the Sejong Institute near Seoul. “The Kim-Trump romance may be short-lived, especially if Trump determines that playing hardball and cornering Kim Jong-un is popular in the run-up to his next election campaign. In Hanoi, Trump learned how desperate Kim is to ease sanctions.”

After the breakdown of the Hanoi talks, Mr. Kim faced growing doubts at home that he could deliver on his promise to revive his country’s troubled economy and keep his long-suffering people from having to “tighten their belt again.”

Mr. Kim may engineer a political purge to keep the elites under control and reassert his authority, Mr. Lee said. He may also reshuffle his negotiating team, replacing Kim Yong-chol, a former spy chief who has led negotiations with Washington, with more seasoned Foreign Ministry diplomats, Mr. Lee said.

“The greatest achievement” of the Hanoi summit meeting “was to clearly prove that there is no potential for convergence between the current North Korean and American trajectories toward North Korea’s denuclearization,” said Ha Young-sun, a prominent South Korean expert on North Korea, in a column posted on Global North Korea, a Seoul-based website specializing in the North.

If North Korea does not correct its course, “the regime will face a second ‘Arduous March,’” Mr. Ha said, referring to a famine in the 1990s that may have killed millions of North Koreans.

It remains difficult to measure how much North Korea is hurt by sanctions. The United Nations has warned that the country is benefiting from sanctions loopholes, conducting ship-to-ship transfers of oil and other banned goods, with the help of Chinese and Russian entities.

But Mr. Kim’s insistence on sanctions relief during his meeting with Mr. Trump suggested that North Korea has begun feeling pains of sanctions in a way it had never before or that Mr. Kim sees sanctions as the biggest obstacle to his ambitious plan to rebuild the economy.

On Wednesday, United Nations and Red Cross officials said that North Korea’s food production last year had fallen to its lowest level in more than a decade, as a prolonged heat wave, along with typhoon and floods, caused the food harvest to drop 9 percent.

Maize yields fell more than 30 percent from average levels in some areas, and rice prices are likely to rise this year, aggravating the food security situation, Margareta Wahlstrom, the president of the Swedish Red Cross, told Reuters.

That has resulted in a “significant food gap,” leaving around 3.8 million North Koreans in urgent need of humanitarian assistance worth $120 million, according to Tapan Mishra, the United Nations resident coordinator in the North, Reuters said.

Since taking power in 2011, Mr. Kim has focused on reviving the economy, allowing more market activities, as well as kicking off a building boom in Pyongyang and in potential tourist zones on the country’s east coast and near its scenic Mount Baekdu. At the same time, he accelerated his country’s nuclear and missile program, proudly announcing in 2017 that he had completed nuclear-armed ICBMs.

But it came with a heavy cost.

Since North Korea conducted its fourth nuclear test in 2016, the United Nations Security Council has drastically tightened the noose around the North Korean economy, banning all of its key exports, such as coal, textiles and fisheries, and limiting its ability to import oil.

After steady growth under Mr. Kim, the country’s economy contracted 3.5 percent in 2017 because of sanctions, according to South Korea’s central bank. It may have contracted 5 percent last year, South Korean economists say.

North Korean exports to China plummeted 88 percent to $210 million last year, according to Chinese customs data. China accounts for more than 93 percent of North Korea’s external trade.

The widening trade deficit with China — it grew 30 percent last year — threatens the country’s hard-currency reserves and its ability to stock its markets and state-run stores with Chinese imports to keep inflation in check, analysts said.

During the Hanoi summit meeting, Mr. Kim made a bold offer by North Korean standards. He offered to dismantle its facilities at Yongbyon, which his country has operated as its main nuclear research and nuclear fuel production complex since the 1980s. In return, he asked Mr. Trump to lift the sanctions imposed since 2016 that have strangled the North Korean economy.

But Mr. Trump demanded that the North do more, seeking a more comprehensive denuclearization agreement, rather than an incremental deal in which Washington give up the sanctions leverage it has built over the years.

“I’d much rather do it right than do it fast,” Mr. Trump said.

His strategy stymies Mr. Kim’s plans to improve the economy and strengthen the legitimacy of his family’s dynastic rule. But time is not necessarily on Washington’s side, Mr. Koh said.

“Sanctions certainly hurt, but they will not make the regime collapse or surrender,” he said. “As talks remain stalemated, the North will keep running its nuclear program and produce more fissile materials for more nuclear warheads.”

Peter Baker contributed reporting from Washington.