Fluoride was introduced into drinking water starting in 1945. The flu vaccine was first made available to the general public a year later. Fuel efficiency standards for cars were adopted in 1975.

Such innovations long ago became stitched into the fabric of American life, largely accepted by most Americans who came to rely on them or gave them little thought. That is, until President Trump and his team came along and began methodically rolling back widespread practices and dismantling long-established institutions.

It should come as no surprise that Mr. Trump would try to undo much of what President Joseph R. Biden Jr. did over the past four years. What is so striking in Mr. Trump’s second term is how much he is trying to undo changes that happened years and even decades before that. At times, it seems as if he is trying to repeal much of the 20th century.

On matters big and small, Mr. Trump has hit the rewind button. At the broadest level, he has endeavored to reverse the globalization and internationalism that have defined U.S. leadership around the globe since World War II, under presidents of both parties. But even at a more prosaic level, it has become evident that Mr. Trump, 79, the oldest president ever inaugurated, simply prefers things the way he remembers them from his youth, or even before that.

He has made clear that he wants to return to an era when “Cats” was the big hit on Broadway, not “Hamilton”; when military facilities were named after Confederate generals, not gay rights leaders; when coal was king and there were no windmills; when straws were plastic, not paper; when toilets flushed more powerfully; when there weren’t so many immigrants; when police officers weren’t discouraged from being rough on suspects; when diversity was not a goal in hiring or college admissions or much of anything else.