WASHINGTON — 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 Donald 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 Trump would try to undo much of what President Joe Biden did over the past four years. What is so striking in Trump’s second term is how much he is trying to undo changes that happened years and even decades before that. It seems as if he is trying to repeal much of the 20th century.

On matters big and small, 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 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.

Just last month, Trump suggested going back to calling the Pentagon chief the “secretary of war,” a title retired in 1947, rather than secretary of defense, a term he dismissed as “politically correct.” Just this month, he again talked about reopening Alcatraz, the famed island prison in San Francisco Bay that was closed in 1963. Just last weekend, he said the Washington Commanders football team should not have dropped the name Redskins, which it did in 2020 amid heightened awareness of racial sensitivities.

Trump suggests that he is on a mission to halt what he considers the degradation of America by “radical left lunatics” and return the country to better times. “We’ve seen some of our political system attempting to overthrow the timeless American principles and other pillars of our liberty, and replace them with some of the most noxious ideas in human history, ideas that have been proven false,” he said.

Trump’s shift into reverse reflects the broader sentiments of many Americans eager for a change in course. The United States has cycled from progressive to conservative eras throughout its history. The liberal period ushered in by Franklin D. Roosevelt eventually led to a swing back to the right under Ronald Reagan, which led to a move toward the center under Bill Clinton.

But Trump has supercharged the current swing. The influential writer William F. Buckley Jr. once defined a conservative as someone standing athwart history and yelling, “Stop!” Trump seems to be standing athwart history yelling, “Go back!”

He has gone further than noted conservatives like Buckley, Reagan, Barry Goldwater or Robert Taft might have imagined possible. While they despised many of the New Deal and Great Society programs that liberal presidents introduced, and sought to limit them, they recognized the futility of unraveling them altogether.

“They were living in an era dominated by liberals,” said Sam Tanenhaus, author of “Buckley,” a biography published last month. “The best they could hope for was to arrest, ‘stop,’ liberal progress. But what they dreamed of was a counterrevolution that would restore the country to an early time — the Gilded Age of the late 19th and early 20th centuries.”

“Trump,” he added, “has outdone them all, because he understands liberalism is in retreat. He has pushed beyond Buckley’s ‘stop,’ and instead promises a full-throttle reversal.”

Indeed, although Reagan vowed during his 1980 campaign to abolish the Department of Education, which had been created the year before over the objections of conservatives who considered it an intrusion on local control over schools, he never really tried to follow through as president, because Democrats controlled the House. The issue largely faded until Trump this year resurrected it and, unlike Reagan, simply ignored Congress to unilaterally order the department shuttered.

“Almost everything Trump has done in his second term, including his ‘one big, beautiful bill,’ has its origins in the conservative ideology of the 20th century,” Tanenhaus said, referring to the sweeping tax and spending legislation the president just signed into law.

While Reagan and Clinton both used the slogan “Make America Great Again,” Trump has made it such a signature mantra that MAGA has become a synonym for his right-wing populist movement.

But Trump has never clearly defined exactly when America was great, and when it stopped being great.

What period of American history is he trying to recapture? The 1950s of his childhood, with its “Leave It To Beaver” era of peace and prosperity, even if women and people of color were still second-class citizens and Sen. Joseph McCarthy blacklisted supposed communists? The 1980s, when Trump was in his New York real estate heyday, featured regularly in the tabloids as he squired models? The early 2000s, when he was a fixture on reality television, barking, “You’re fired!” every week?

Or is he looking even further back, to an era before he was even alive? In recent months, the president has talked repeatedly about the Gilded Age as a halcyon period in American history, highlighting William McKinley (“the Tariff King”) as his presidential model. “Our country was the wealthiest, proportionately the wealthiest, from 1870 to 1913,” Trump said at a Cabinet meeting last week, suggesting a return to a time when tariffs were high and the income tax was yet to be enacted.

He seems to be reflecting in part the view of people like Howard Lutnick, the billionaire former Cantor Fitzgerald chief executive serving as Trump’s commerce secretary. At a Trump rally at Madison Square Garden before the November election, Lutnick expressed nostalgia for the time when robber baron capitalist tycoons dominated the national economy.

“When was America great?” Lutnick said. “At the turn of the century. Our economy was rocking. This is 1900, 125 years ago. We had no income tax and all we had was tariffs. All we had was tariffs. And we had so much money that we had the greatest businessmen of America get together to try to figure out how to spend it. That’s who we were then.” America went downhill, he said, when it had to rebuild the globe after two world wars.

The wistfulness for a time before living memory, of course, is based on a selective reading of history. By traditional measures, most Americans are far better off now than they were in 1900, when typical workers made a fraction of today’s salaries and lived without running water, indoor plumbing, electricity or modern medicine. Neither women nor people of color enjoyed full rights of citizenship, much less the chance to become one of those “greatest businessmen.”

Tariffs, which went out of favor in the Great Depression, are the leading example of Trump’s efforts to return to a previous way of doing things. But he has moved to abolish many of the institutions and legal frameworks of the past century, calling them wasteful bureaucracies, impediments to growth or examples of liberal excess.

Geoffrey Kabaservice, vice president of political studies at the Niskanen Center, a Washington research institution, said Trump was tapping into a sense of unease among many Americans that the country had gone too far and too fast in some areas.

“He’s appealing to a widespread sense among Americans, particularly Republicans, that the 1950s were simpler and better times, when the country was united, proud and optimistic in ways that it no longer is now,” said Kabaservice, the author of books on liberalism and conservatism.

“It’s a vision,” he added, “that all but erases the complexity of the era — McCarthyism, the rise of television, the nascent civil rights movement, fears of nuclear war, etc. — but correctly senses that working-class values and institutions — family, neighborhood, church, union — loomed much larger then than they do now.”

But if a vision of a simpler past appeals to him, even Trump is discovering that some of what he wants to roll back may have some value after all. Not that long ago, he had declared that he planned to “phase out” the Federal Emergency Management Agency and return disaster response to the states. As he visited Texas on Friday to offer support following last weekend’s deadly floods, however, his advisers emphasized that he planned to reform the agency, not do away with it.