At one time or another, Trump has attacked the judiciary, ridiculed the media, defended torture, condoned police brutality, urged supporters to rough up hecklers and — jokingly or not — equated mere policy disagreements with treason. He tried to undermine faith in America’s electoral process through a bogus advisory commission on voter integrity. He routinely vilifies federal law enforcement institutions. He libels immigrants and the countries from which they come. His words are so often at odds with the truth that they can appear ignorant, yet are in fact calculated to exacerbate religious, social and racial divisions. Overseas, rather than stand up to bullies, Mr. Trump appears to like bullies, and they are delighted to have him represent the American brand. If one were to draft a script chronicling fascism’s resurrection, the abdication of America’s moral leadership would make a credible first scene.

Equally alarming is the chance that Mr. Trump will set in motion events that neither he nor anyone else can control. His policy toward North Korea changes by the day and might quickly return to saber-rattling should Pyongyang prove stubborn before or during talks. His threat to withdraw from the 2015 Iran nuclear agreement could unravel a pact that has made the world safer and could undermine America’s reputation for trustworthiness at a critical moment. His support of protectionist tariffs invites retaliation from major trading partners — creating unnecessary conflicts and putting at risk millions of export-dependent jobs. The recent purge of his national security team raises new questions about the quality of advice he will receive. John Bolton starts work in the White House on Monday.