The national security adviser is the person who makes sure the president hears the views of all the national security agencies, including the State Department and the Defense Department, and who drives policy toward a decision. It is hard to see Mr. Bolton acting as an honest broker. He is known to play a ruthless inside game as he maneuvers to win bureaucratic battles and freezes out people he thinks have crossed him. He has been such a lightning rod that he couldn’t get confirmed as United Nations ambassador in 2005, so President George W. Bush gave him a recess appointment, and he stayed in the job about a year. It was considered unlikely that the Senate would confirm him as secretary of state, but the national security adviser job doesn’t require confirmation.

Bringing on the fiery Mr. Bolton now, at a delicate moment with North Korea, is a terrible decision. While Mr. Trump has often threatened North Korea with military action, he accepted Mr. Kim’s invitation to a summit meeting, brokered by South Korea’s president, who is eager for a diplomatic solution to the nuclear crisis.

Mr. Bolton, by contrast, told Fox News earlier this month that talks would be worthless, and he has called South Korean leaders “putty in North Korea’s hands.” On Feb. 28, he insisted in a Wall Street Journal op-ed article that “it is perfectly legitimate for the United States to respond to the current ‘necessity’ posed by North Korea’s nuclear weapons by striking first.” Last summer he wrote in The Journal, “The U.S. should obviously seek South Korea’s agreement (and Japan’s) before using force, but no foreign government, even a close ally, can veto an action to protect Americans from Kim Jong Un’s nuclear weapons.”

On Iran, Mr. Bolton and the president are in sync, with both arguing that the United States should withdraw from the nuclear agreement by a May deadline. In March 2015, he argued in a New York Times Op-Ed opinion article that only military action like Israel’s 1981 attack on Saddam Hussein’s Osirak reactor in Iraq or its 2007 destruction of a Syrian reactor “can accomplish what is required.”

Going to war in either of these cases would not only create unnecessary bloodshed; it would be disastrous for the United States and its allies, South Korea and Japan. The Iran deal has substantially halted the nuclear program and needs to be maintained. Negotiations between the United States and North Korea, given a new impetus by Mr. Trump and Mr. Kim, need to be tested.

Mr. Bolton’s position on Russia, that NATO must have a strong response to the Kremlin-linked poisoning of a former Russian spy in Britain, is somewhat better than Mr. Trump’s. But his rejection of a two-state solution to the Israeli-Palestinian conflict and his endorsement of a book by the anti-Muslim activist Pam Geller are unacceptable positions for a top American official.

Mr. Bolton is certain to accelerate American alienation from its allies and the rest of the world. Congress may not be able to stop his appointment, but it should speak out against it and reassert its responsibilities under the Constitution to authorize when the nation goes to war.