Losing face might prove a blow for Mr. Kim, who cultivates the image of a peerless leader in his totalitarian homeland.

Lee Byong-chul, a senior fellow at the Institute for Peace and Cooperation in Seoul, the South Korean capital, said Mr. Trump might be content with a freeze on North Korean weapons tests. “But I wonder how long the moratorium will last,” he added.

As Mr. Trump left Hanoi aboard Air Force One on Thursday, the path forward was unclear.

At a news conference on Thursday, Mr. Trump said that his relationship with Mr. Kim remained “very friendly” and that he intended to continue negotiations at a measured pace. But during a surprise late-night news conference in Hanoi, Choe Son-hui, a vice foreign minister of North Korea, warned that Mr. Kim had “lost some of his desire to negotiate with the United States.”

Mr. Kim has options other than reigniting nuclear and ballistic tests. He could choose to wait, particularly given that Mr. Trump is likely to be sidetracked by his own domestic troubles for the foreseeable future.