News of John Bolton’s appointment as national security adviser, displacing General H.R. McMaster, landed like a bomb in Foggy Bottom. Washington’s usually staid diplomatic corps was already on edge following the shock expulsion of Rex Tillerson, soon to hand the State Department over to somewhat trigger-happy C.I.A. Director Mike Pompeo. Bolton, however, is his own unique breed of hawk. “Horrified has been the ongoing feeling. Many recall his poor tenure at the U.N. [and] as undersecretary,” a current senior State Department official told me, reflecting on Bolton’s controversial résumé. While serving under Secretary of State Colin Powell, Bolton was both a cheerleader and early architect of the Iraq war. “General expectations keep falling.”

Bolton’s ascension, like Tillerson’s downfall, was seen by many as an inevitability, as Donald Trump reconfigures his inner circle to better reflect his nationalistic impulses. A veteran of the George W. Bush administration with an infamous fondness for military adventurism, Bolton reportedly made an impression on Trump with his frequent diatribes on Fox News, railing against the Iran nuclear deal and calling for more aggressive posturing toward North Korea. “Bolton was always going to end up in the Trump White House sooner or later. His world view, or more accurately, anti-world view, is vastly closer to Trump’s than most of the president’s existing advisers,” said Richard Gowan, a U.N. expert and professor at Columbia University’s School of International and Public Affairs. “He loves to talk about U.S. strength, and he is a notoriously aggressive negotiator. Those are precisely the qualities Trump looks for, and the two men fully deserve one another.”

Denizens of the Deep State fear Bolton for mostly predictable reasons. A nationalist with little patience for the pragmatic realism of the G.O.P.’s old guard or the democracy promotion that defined the early Bush years, Bolton is expected to tell Trump what he has been waiting to hear: that he should ditch the J.C.P.O.A. binding the United States to the Iran nuclear framework, and impose harsh sanctions on Tehran. “Bolton is not interested in [a deal],” a Senate aide told me. “It’s like the perfect storm with where the president has been on it. He has his yes-man.” (Bolton famously wrote a 2015 op-ed titled “To Stop Iran’s Bomb, Bomb Iran.”) Others worry that Bolton could be a de-stabilizing force amid the North Korea crisis, sabotaging the potential for high-level talks between Trump and Kim Jong Un. “Bolton, Pompeo, and [Nikki] Haley could turn into a very hawkish triumvirate running the U.S. foreign-policy machine,” Gowan said. “They are likely to be rivals behind the scenes, but on issues like Iran they are actually all certified hard-liners.”

On a more prosaic level, however, there is the fear that Bolton will succeed in navigating the Swamp where so many other Trump advisers have failed. “Everyone that knows him knows that he is very smart, and not only is he very smart, but he is very effective,” a former senior U.S. official cautioned. “Bolton is so dangerous because he is good at what he does.” Ian Bremmer, a foreign-policy analyst and founder of the Eurasia Group, described Bolton as “extremely bureaucratically capable”—an infighter with sharp elbows and a tactile understanding of the media. “He knows how to get in meetings that matter, he knows how to play the media, and he is going to push hard to be a very strong national security adviser with those views vis-à-vis his colleagues in Cabinet.”