SEOUL, South Korea — 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.