But Mr. Bolton is also a strong supporter of the NATO alliance and a much harsher critic of Russia and President Vladimir V. Putin than is Mr. Trump, Mr. Janning noted. That point may help protect the president from suggestions that he is “soft on Russia.”
“Bolton is relentless, intelligent and effective,” said François Heisbourg of the International Institute for Strategic Studies, who as a French military analyst dealt with Mr. Bolton during the administration of George W. Bush. “But he’s not a neoconservative and has no interest in democracy promotion. He is a man of the Trumpian world — no allies, no multilateralism.”
Stephen Bush of the center-left British magazine New Statesman said that Mr. Bolton was “the man who makes neoconservatives say, ‘Steady on, old chap.” ’
Nigel Sheinwald, a former British ambassador to Washington and to the European Union, also dealt with Mr. Bolton on Iran and arms control.
“He tried to push Bush policy in a much more extreme direction,” Mr. Sheinwald said. “Given that the U.S. and the U.K. had so much at stake together, he was oddly deaf to the idea that America had allies and was very critical of the U.K. in almost everything we did,” especially over Iran.
Mr. Bolton’s ascension promises strains with American allies both in Europe and in Asia — first over the nuclear deal with Iran and then over the nuclear capacity of North Korea.
The crisis most expect to arise first concerns Iran, with both Mr. Bolton and the new secretary of state, Mike Pompeo, sharing Mr. Trump’s view that the 2015 Iran nuclear deal, negotiated by the United States the other permanent members of the Security Council and Germany, should be renegotiated or scrapped. With the departure of Secretary of State Rex W. Tillerson and of Mr. Bolton’s predecessor, Lt. Gen. H. R. McMaster, Defense Secretary Jim Mattis is a more lonely voice arguing that the deal, however flawed, is better than any alternative.
Mr. Bolton’s opposition to the negotiations with Iran is longstanding, dating from his days in the Bush administration, when he helped derail European talks with Tehran at a time it still had only a few centrifuges, Mr. Heisbourg said.
“Bolton wanted us to fail,” Mr. Heisbourg said. “Had we gotten into serious negotiations 10 years before, the Iran deal would be very different, and we could have had the deal people like Bolton say they want now.”
If Mr. Trump withdraws from the Iran deal in mid-May and imposes new American sanctions, as now seems very likely, the other signatories to the agreement, especially the Europeans, will have a difficult choice. They can try to protect the deal, in defiance of Washington. Or they can blame its failure on Mr. Trump, try to draw Iran into another round of talks and threaten further sanctions themselves if Iran resumes enrichment of uranium to military grade.
While Mr. Bolton’s appointment was welcomed in Israel, with the education minister, Naftali Bennett, calling him “an extraordinary security expert, experienced diplomat and a stalwart friend of Israel,” the official Iranian response called him a “supporter of terrorists” now in “the highest political position in Trump’s totalitarian government.”
The appointment of Mr. Bolton has set teeth on edge in Asia, where American allies are highly anxious about a developing nuclear crisis that appears all but inevitable. Mr. Bolton, Mr. Pompeo and Mr. Trump all say that North Korea could face pre-emptive warfare if it does not agree to dismantle its nuclear weapons.
“People are trying to avoid appearing terrified,” said Tong Zhao, a nuclear policy expert at the Carnegie-Tsinghua Center for Global Policy in Beijing. “But people are deeply concerned.”
He noted that Mr. Bolton’s appointment followed the imposition of tariffs, a Nuclear Posture Review and other steps that signal a worrisome deterioration in relations as Washington increasingly treats China as a strategic competitor.
In that respect, Shi Yinhong, a professor of international relations at Renmin University in Beijing, said that the choice of Mr. Bolton, while a surprise, would not change an already antagonistic tone. “Almost everyone in the Trump administration now takes a harsh or hard-line posture toward China,” he said. “I think Bolton will not be an exception.”
Lee Byong-chul, senior fellow at the Institute for Peace and Cooperation in Seoul, said that South Korea must now manage its “very bad chemistry” with Mr. Bolton, “who is all about sticks.”
Mr. Bolton has derided South Korea for trying to play peacemaker with Pyongyang, saying the South was “like putty in North Korea’s hands.”
“We will have to see if Bolton opens his mouth and launches his verbal attacks against the North,” Mr. Lee said. “That will give North Korea an excuse to step away from its summit proposal. The Trump-Bolton team then will ramp up pressure. And we will hear more talk about a pre-emptive strike and see tensions rising again on the Korean Peninsula.”
Others thought he might temper his words, but China would still worry about Mr. Bolton having Mr. Trump’s ear, said Chen Dingding, a professor of international relations at Jinan University in Guangzhou, China.
“He’s a hard-liner, not just toward China but to the whole world,” Mr. Chen said. “North Korea, Iran, the European Union, the United Nations — every side — it’s not just China. But he does represent a worldview of the Trump administration, one of ‘America First’ and unilateralism over multilateralism. I think the whole world should be concerned, not just Asia.”
For Xenia Wickett, a former official at the National Security Council who now directs the United States and Americas Program at the London research organization Chatham House, Mr. Bolton “knows his portfolios and is eminently qualified for the job.” But the concern, she said, is that he “is extremely far toward the hawkish end of the national security world.”
A good national security adviser, who does not need confirmation by Congress, has a strong relationship with the president, with his own team and with Congress, she said. Mr. Bolton will have “a good relationship with Trump, a mixed relationship with his staff and a not very good relationship with Congress,” she said, suggesting that even Republicans there would have been unlikely to confirm him had that been required.
The best national security advisers coordinate policy more than they run it, she said, wondering: “Can Bolton do the coordination thing?”