THE PRESIDENT’S national security adviser is meant to coordinate policy formation inside the administration, manage disagreements among agencies and tee up important decisions, while separately advising the president of his or her own views. John Bolton, whom President Trump has said will take over the position next month, is unsuited for that role. His record is that of a rigid, bombastic ideologue with a history of bullying colleagues and twisting intelligence. His advocacy of extreme policies, including preventive war against North Korea and Iran, could lead Mr. Trump and the country to catastrophe.

It’s not clear whether Mr. Trump chose Mr. Bolton because he agrees with his views or simply because he likes watching his commentaries on the Fox News network. (The president is said to have set aside previous plans to recruit the former U.S. ambassador to the United Nations because he did not like the look of his bushy mustache.) Whether he intends it or not, Mr. Trump is likely reintroducing conflict and chaos into his national security team, which under fired national security adviser H.R. McMaster and Secretary of State Rex Tillerson approached competence.

Mr. McMaster, who joins the growing list of honorable public servants whose reputations have been besmirched by Mr. Trump, managed to purge the National Security Council staff of unqualified personnel imported by his predecessor, Michael Flynn, and to attract some foreign policy professionals. Now they will have to weigh Mr. Bolton’s well-known history, which includes attempting to pressure State Department intelligence professionals into accepting the false conclusion that Cuba had biological weapons and harassing an Agency for International Development whistleblower. USAID contractor Melody Townsel told the Senate Foreign Relations Committee in 2005 that Mr. Bolton chased her through a Moscow hotel in 1994 “throwing things at me, shoving threatening letters under my door and, generally, behaving like a madman.”

Republican senators who studied this record refused to confirm Mr. Bolton as U.N. ambassador, and it is likely that he would have been rejected by the present Senate if nominated to a position requiring confirmation. Mr. Trump gets to choose his own national security adviser, but in selecting Mr. Bolton he is inviting not just staff turmoil but a radical turn in his foreign policy.

Mr. McMaster, along with Mr. Tillerson and Defense Secretary Jim Mattis, persuaded Mr. Trump to put off some reckless moves, such as canceling the nuclear deal with Iran and withdrawing U.S. troops from Afghanistan. Now the president’s chief foreign policy adviser will be encouraging his inclination to breach the nuclear accord — something that would alienate U.S. allies and invite a conflict with Iran.

Mr. Bolton’s public advocacy of war with North Korea could sink Mr. Trump’s already risky plan to meet with regime leader Kim Jong Un. If Mr. Trump were to heed Mr. Bolton’s argument that a preemptive strike is preferable to managing the nuclear threat, East Asia and perhaps even the U.S. homeland could be exposed to unthinkable bloodshed.

We’d suggest that GOP leaders in Congress weigh in against Mr. Bolton, but they have shown again and again that they are unwilling to challenge the White House. That leaves the hope that Mr. Mattis and other competent officials remaining in the administration will balance what is likely to be a provocative influence on the president.