The cautious and considered HR McMaster is leaving the White House to be replaced by one of the most polarizing, irascible figures operating in contemporary national security circles: Former UN Ambassador John Bolton.

Bolton, who recently penned an op-ed in the Wall Street Journal arguing that a preemptive strike against North Korea would be both legal AND desirable, is widely believed to be one of the most interventionist figures to ever hold a senior position in the US government. Case in point: Before 9/11, Bolton helped found a group calling for the unilateral overthrow of Saddam Hussein. Rand Paul declared that Trump was wrong to trust someone who is "unhinged as far as believing in absolute and total intervention."

So it should hardly come as a surprise that Bolton plans to shake up the National Security Council staff when he arrives in the West Wing. Foreign Policy reports that Bolton, Trump's third NSA in 15 months, is preparing to begin firing staff and replacing them with his own allies, as well as a few allies of former NSA Michael Flynn, who share Bolton's hawkish views.

As one might expect, the Obama holdovers and McMaster loyalists will be the first to go. But they won't be the only ones: Those targeted for removal include officials believed to have been disloyal to President Donald Trump - especially those who have leaked about the president to the media.

"Bolton can and will clean house," one former White House official said. Another source said "He is going to remove almost all the political [appointees] McMaster brought in." A second former White House official offered a blunt assessment of former Obama officials currently detailed or appointed to the NSC: "Everyone who was there during Obama years should start packing their shit."

The circumstances surrounding McMaster's departure will only embolden his successor to make sweeping changes. As FP reports, McMaster was reportedly planning to hang on for a few more months, but a recent leak about Trump's decision to congratulate Russian President Vladimir Putin.

McMaster’s departure may have been hastened by leaks emanating from the White House. Two sources familiar with the matter said McMaster was going to stay on until early summer. But when the Washington Post reported this week that Trump had congratulated Putin in a phone call on his fraudulent election win — after receiving written briefing materials from the NSC instructing him not to congratulate Putin — the president reacted furiously and blamed McMaster. The story caused Trump to speed up McMaster’s departure, the sources said.

Bolton is already in talks with certain longtime advisors and is likely preparing to offer several of them jobs in the West Wing. One such advisor is Matthew Freedman, a Republican consultant who previously advised Bolton at the State Department and the United Nations. Freedman and many other Bolton allies are pushing the incoming national security advisor to make sweeping changes (changes that will, of course, benefit them).

On Thursday evening, just hours after Trump tapped him for the job, Bolton held a call with longtime advisors, including Matthew Freedman, a Republican consultant who once advised Bolton at the State Department and the United Nations. Freedman is currently helping manage the transition, according to a source familiar with the call. "Freedman is a very political guy that Bolton likes," one Republican source said. "He is overly ambitious about cleaning house." Freedman disputed that account, saying he was not aware of the Thursday phone call. "I can tell you there is no list," he said. Another source close to Bolton said it was premature to be talking about personnel changes.

While Trump might object to Bolton's mustache (the president has a distaste for men with facial hair), the two at least see eye to eye on policy issues. In a way, Bolton could be considered a "proto-Trumpian" figure due to his criticism of the United Nations and the European Union - positions that Trump has also embraced.

Bolton's friends believe this closeness will allow Bolton to make swift changes at the White House. Indeed, Bolton's allies already have two names that should be at the top of Bolton's list of staff to fire: Deputy NSA Nadia Schadlow and former McMaster deputy Ricky Waddell.

Among the officials Bolton’s allies are urging him to fire is Nadia Schadlow, currently the deputy national security advisor for strategy. Schadlow was the primary author of the administration’s recently released National Security Strategy, which was viewed as a surprisingly mainstream document that reaffirmed many traditional U.S. foreign-policy positions. Another official likely to be targeted in a Bolton purge is McMaster’s deputy, Ricky Waddell. It wouldn’t be the first purge to follow a change in Trump’s national security advisor. When Lt. Gen. H.R. McMaster replaced retired Lt. Gen. Michael Flynn in the job last year, McMaster systematically eliminated officials seen as loyal to his predecessor. According to four sources close to the White House, those so-called "Flynnstones" - advisors loyal to Flynn - are believed to be plotting their return to the NSC. Whether Bolton will sign off on the staff purge his allies and advisors are pushing is less clear, though he has been insistent about ousting so-called Obama holdovers. "You could easily say that people close to Bolton want these people to go," one source said. Other sources stress that Bolton, a veteran bureaucratic infighter, makes his own decisions. A source close to Bolton cautioned that any staffing changes would take time, given the need to process security clearances. That means Bolton will likely be stuck with his current staff for the May summit meeting between Trump and North Korean leader Kim Jong Un.

Trump and Bolton have reportedly discussed staffing changes since at least last July, when Bolton was offered the job as McMaster’s deputy - a position currently held by Waddell. Trump told Bolton that the deputy job would lead to the top post, but Bolton declined, saying he’d rather wait until he was offered the national security advisor job.

However, there are two factors that Bolton's allies believe could make life difficult for the former ambassador to the UN.

One is his hawkishness toward Russia - which puts him at odds with Trump (though Trump, who is planning to expel dozens of Russian diplomats over the Skripal incident).

Another is whether he manages to get along with Chief of Staff John Kelly - very much a supporter of the establishment view of American foreign policy. Bolton also has Trump's ear, which could lead to tensions between the two men. Since he arrived in the West Wing, Kelly has proven incredibly effective at keeping his job, and has helped dispatch many West Wing rivals.

Bolton will need to tread carefully if he wants to outlast his two predecessors.