The new team unites two of the most virulent opponents of the 2015 Iran deal: Mike Pompeo, the C.I.A. director who was nominated as secretary of state after Mr. Tillerson was unceremoniously fired last week, and Mr. Bolton, a former ambassador to the United Nations who famously declared two decades ago that if 10 stories of the organization’s headquarters disappeared, “it wouldn’t make a bit of difference.”
Even before the announcement on Thursday night that General McMaster was leaving the White House — following months in which Mr. Trump chafed at his briefings and complained that his national security adviser was “managing” him — it seemed likely that the president would follow his instincts and abandon the Iran deal.
Even the chairman of the Senate Foreign Relations Committee, Senator Bob Corker, Republican of Tennessee, said over the weekend that he expected the president would “move away from it.”
Now that seems inevitable.
If so, this new team of hard-liners will be cast, in their first month together, into two of the most volatile nuclear issues of the post-Cold War era. The question is whether, once in office, they will manage America’s allies and adversaries by ultimatum or by diplomacy.
There is little in Mr. Bolton’s past, or in Mr. Pompeo’s, to indicate they have much time for the give-and-take of the diplomatic process, where to get something the United States also gives something.
“A president who is under political siege and faces at least two extremely sensitive situations in the world has picked the hawkish team that is most likely to turn those into conflicts,” said David Rothkopf, who has written two histories of the National Security Council.
“We get in trouble when the national security apparatus presents a single view, rather than a range of views,” Mr. Rothkopf said.
The last time there was this conservative a clique around the president was 15 years ago, when Vice President Dick Cheney and Defense Secretary Donald H. Rumsfeld pressed President George W. Bush to invade Iraq. At that time, Secretary of State Colin L. Powell played the role that Mr. Mattis plays now: the voice of caution about what happens when unpredictable wars start, and the unanticipated follows.
The anniversary of the beginning of the Iraq invasion was Wednesday. Many who were involved in that decision have since begun to acknowledge the flaws — not only the reliance on what turned out to be false intelligence, but false assumptions about how quickly peace and prosperity would come to Iraq.
Mr. Bolton continues to assert that it was the right decision and was among those who insisted, in the run-up to the war, that Saddam Hussein was on the brink of getting weapons of mass destruction. His initial instinct was to use force.
In North Korea’s case, there is no debate: The country has conducted six nuclear tests, and has demonstrated that its missiles are within striking distance of American cities. If his argument for pre-emptive strikes was convincing to Mr. Bush, he can make a far more persuasive case to Mr. Trump.
It is unclear how Mr. Pompeo and Mr. Bolton will jell, or how the dynamic may be changed by another new player in the national security orbit around Mr. Trump: Gina Haspel, his nominee for director of the C.I.A.
Hers may be the most difficult nomination to succeed, because of her history overseeing part of the “enhanced interrogation” program after the Sept. 11, 2001, attacks and the C.I.A. move to destroy tapes of those interrogations. A career intelligence officer, Ms. Haspel will now be joining the policymakers — though not in a policy role, if she follows the tradition of offering intelligence assessments rather than recommendations.
Mr. Bolton, the third national security adviser in 14 months, is taking over a National Security Council that, a year ago, leaned a lot further to the right.
General McMaster replaced many of the appointees left over from the 24-day tenure of Michael T. Flynn, whose instincts, especially on Iran, hewed closer to Mr. Bolton’s. Mr. Flynn recently pleaded guilty to one count of lying to the F.B.I. about his contacts with Russia’s ambassador to the United States during the presidential transition.
North Korea and Iran aside, what is fascinating about Mr. Bolton’s selection is that his operating style runs contrary to exactly the kind of national security adviser the Republican establishment has argued that a president who is inexperienced in foreign affairs needs.
The model, those establishment figures declare, was Brent Scowcroft, the former Air Force general who served Gerald R. Ford and George H. W. Bush. Mr. Scowcroft viewed the job as one designed to bring together views — and consensus if possible — to be sure that the president heard a variety of opinions.
General McMaster, a historian, seemed to follow the Scowcroft model a bit closer. He presented the president with a variety of options, often with lengthy explorations about the pros and cons of each. Mr. Trump had no patience for that kind of briefing.
The president is unlikely to get the same kind of briefings from Mr. Bolton, who in every job he has held has made his own views clear, and sometimes silenced those who disagreed.
In the coming months, that person could likely be Mr. Mattis, more so than other players left in the mix. The defense secretary has often called war on the Korean Peninsula unthinkable, given the likely casualties. He has told Congress that his advice to the president is to stay in the Iran deal, despite its imperfections, because all the alternatives are worse.
His may be a lonely view.