The appointment “adds to the perception of the U.S. basically being a headless chicken, which is bad for the world,” said Koichi Nakano, a political scientist at Sophia University in Tokyo.
The initial reaction from South Korea’s government was muted, a reflection of the delicate dance the country’s president, Moon Jae-in, is attempting as he tries to draw the United States toward engagement with North Korea.
A senior aide to Mr. Moon tried to play down fears that Mr. Bolton’s hawkish stance on the North could derail the latest efforts at dialogue, saying that Mr. Trump himself — not his aides — was pushing for a summit meeting with Mr. Kim.
The adviser, who spoke on condition of anonymity given the diplomatic sensitivities, said that Mr. Bolton would be a trusted adviser to Mr. Trump and that South Korea planned to consult with him as it sought to resolve tensions on the Korean Peninsula.
Mr. Bolton has derided South Korea for trying to play peacemaker with Pyongyang, saying the South was “like putty in North Korea’s hands” and that “there’s a sucker born every minute.”
There was no immediate reaction from North Korea, which has recently refrained from its usual bellicose attacks on the United States. In the past, the North has refused to deal with Mr. Bolton, calling him “human scum” and a “bloodsucker.”
When Mr. Bolton was an under secretary of state in the Bush administration, he called the North’s government “tyrannical” and described the lives of ordinary North Koreans as “a hellish nightmare.”
Japan’s foreign minister, Taro Kono, who recently met with General McMaster, said that he was surprised by the timing of Mr. Bolton’s appointment, but that it should not alter the course of Prime Minister Shinzo Abe’s foreign policy.
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“Our stance is completely coordinated with the U.S. government so I believe there is no change of direction,” he told reporters on Friday.
Japan has much at stake in the Trump-Kim talks, as North Korea’s missiles can easily reach Tokyo and have already flown over Japan.
Mr. Nakano said some hard-line supporters of Mr. Abe would welcome the appointment. Mr. Bolton’s “hawkish stance on just about everything would be more in sync with the previous line that the Abe government has been pursuing, particularly on North Korea,” he said.