“I heard you’re actually the devil incarnate and I wanted to meet you,” Defense Secretary James Mattis said as he greeted John Bolton at the Pentagon on Thursday. Mattis was joking, of course, but it appeared that he was also trying to downplay speculation that he, and other members of Donald Trump’s security and foreign-policy teams, might clash with the president’s new national security adviser—a notorious Bush-era war hawk. Bolton, a former undersecretary of State and ambassador to the U.N., can evince an avuncular, Muppet-like quality, with his overgrown eyebrows and Yosemite Sam mustache. Inside the State Department, however, former colleagues warn that his abrasive personality is matched only by a tactile, nearly algorithmic understanding of the levers of power. It’s a form of wonkery that could prove particularly valuable in his new role, as the fulcrum between various intelligence and diplomatic agencies and the president, himself. “I think he knows the interagency better than anyone in the cabinet, beyond probably the vice president,” Andrew Bowen, who worked with Bolton at the American Enterprise Institute, told me. A former U.S. official put it more succinctly: “Bolton is so dangerous because he is good at what he does.”

Bolton’s arrival comes at a critical juncture for the presidency. The recently departed H.R. McMaster and Rex Tillerson helped provide Mattis with a bulwark against Trump’s worst foreign policy instincts. “We had that period of 14 months where Rex Tillerson was doing his own thing, and McMaster was doing his own thing,” Bowen continued. “There never was any cohesiveness—which I think oftentimes meant that there was not the president’s ambition and agenda was never actually being fully carried out, because there [was] all this competing chaos.” Bolton, however, is a member of a new coalition of administration officials that share the president’s nationalistic and pugilistic ideology. An unrepentant early architect and cheerleader of the U.S. invasion of Iraq, he has been a fierce advocate of ditching the Obama-era Iran nuclear accord, and has previously called for a regime change in North Korea. His appointment, alongside the nomination of the comparably hawkish Mike Pompeo at the State Department, portends the most cohesive approach to U.S. foreign policy since Trump took office. “I think it’s gonna be a much better-run machine,” Bowen said.

While sources I spoke with dismiss the swirling narrative that Bolton will “clean house” upon his arrival at the N.S.C., they do expect him to re-assert its authority, which eroded as McMaster fell out with Trump. “It probably will be playing a much stronger role than it has under McMaster, who didn’t have great rapport with the president, or [Mike] Flynn, who flamed out immediately,” Matthew Waxman, a Columbia University professor who held various positions in the Bush administration, said. “Bolton is sure to make sure that it has a significant role to play and he’s very skilled at doing that.”

One former U.S. official, who held positions in both the Ronald Reagan and George W. Bush administrations, told me that he believes Bolton will effectively enable the president. “One of the main functions of the N.S.C. is to make sure that presidential decisions are implemented by the government, which if you think about it, is not automatic. There are these gigantic bureaucracies and the president is one person,” this person said. “I think where John will be more active is in any situation where he believes that a decision that the president has made is not being implemented, or if information is being held back from the White House. That is where the N.S.C. really needs to be most forceful.”