What Donald J. Trump promised in his first term in office was America First. What he delivered, as his allies, adversaries and many of his former aides remember it, was a complete breakdown in foreign policymaking.

Most of what Mr. Trump has said in his campaign to return to office suggests that in a second term, he plans more of the same, that he considers unpredictability to be his signature weapon. He revels in it, telling The Wall Street Journal’s editorial board in October that he wouldn’t have to threaten China with the use of American military force over Taiwan because President Xi Jinping “respects me and he knows I’m crazy,” using an expletive before “crazy” for emphasis.

Foreign policy experts often cast the choice Americans will make next week as one that will decide whether America moves back toward isolationism or stays with some version of President Biden’s alliance-building, internationalist approach.

That is partially true: If Mr. Trump is defeated, his single term in office could very likely be viewed in history as a blip in America’s post-World War II approach to the world.

If Ms. Harris loses, however, it would mean that Mr. Biden’s term was the definitive end of an era in which the United States was a reliable guarantor of Western security.

Mr. Trump was never a true isolationist, of course, and for all his internationalist talk Mr. Biden has demonstrated more than a few streaks of nationalism. But should Mr. Trump prevail, it will almost certainly mark a return to an era of foreign policy decrees, untethered to any policy process, at a moment of maximum international peril.