Depopulation — unremitting, long-term population decline — promises to be the 21st century’s most important demographic trend.

After centuries of seemingly unstoppable increase, world population is on track to peak soon.

The United Nations’ latest projections envision depopulation starting as soon as 2052 — just a generation hence.

Depopulation is no future fantasy. A growing number of countries are in prolonged population decline.

China, Japan, South Korea, Taiwan — all already depopulators, as are Russia and eight of the European Union 27.

By UN estimates, in fact, more than 50 countries and territories around the world had fewer births than deaths in 2023.

For such “net mortality” societies, only immigration can prevent national-population shrinkage.

Once upon a time, depopulation would have been unthinkable for the United States.

No longer. And it could happen much faster than almost anyone realizes.

The demographic engines that have powered America’s amazing economic and geopolitical ascent since 1776 — fertility and immigration — are faltering today.

Let’s look at both, starting with fertility — the foundation of every country’s demographic outlook.

Throughout our history, American birth rates have been exceptionally high for a rich nation. In colonial times, total fertility was about 7 births per woman; Benjamin Franklin likened our frontiersmen to “locusts swarming across the countryside.”

More recently — over the postwar era’s past three generations — US fertility levels averaged 20% above Europe’s and 30% above Japan’s.

But falling birth rates pushed developed countries’ fertility below the replacement level 50 years ago — including America’s.

In the years since 1972, US annual fertility rates only hit replacement twice.

For most of this period (1972 to 2007) American childbearing was only just barely below replacement.

But in 2008 it started to slide, and by the early 2020s it was more than 20% below the roughly 2.1 births per woman required for long-term population stability.

At 1.6 births per woman today, US fertility remains higher than in virtually any other rich country.

But with such low levels of childbearing, it’s only a matter of time before a permanent surfeit of deaths over births becomes part of (so to speak) the American way of life.

Between 2007 and 2023, “natural increase” in America — births minus deaths — plunged, from about 1.8 million to just over half a million. So when will “natural increase” go negative, with deaths exceeding births?

The Census Bureau expects the crossover to occur in 2038 — just 13 years from now. Then the gap between deaths and births would steadily widen.

But “net mortality” might come to America much sooner.

The latest UN “low variant” scenario conjectures we could hit it next year, presupposing 1.4 births per woman — around the European Union’s level today.

America is not there — yet. But any further fertility declines will only hasten America’s slump into the ranks of “net mortality” societies.

Once deaths exceed births, population must decline — unless immigration prevents it.

Immigration has played a special role in America’s demographic history: It’s the main reason US population is more than 100 times higher today than at the Declaration of Independence.

Thanks to immigrants and their descendants (migration’s “compound interest”), the United States is the world’s third-most-populous country, behind only India and China.

Our national myth maintains we are “a nation of immigrants,” and there is a deep truth in that up to today.

Our population’s foreign-born share may be higher than at any previous point in our history — even the 1890s.

But our mythos overlooks the fact there were long decades — indeed generations — in which Americans severely restricted immigration.

From World War I’s outbreak until the mid-1960s immigration-reform legislation strikingly few foreigners were allowed into the United States.

The year 1970 marked the all-time low for our population’s immigrant share.

The 50-year immigration clampdown was a reaction against the huge foreigner influx during America’s first Gilded Age.

That unregulated inflow of low-skilled immigrants depressed wages for less-educated native-born Americans, exacerbated income and wealth gaps in the country and stimulated anti-immigration sentiment.

Sound familiar?

Until very recently, postwar American opinion has been generally pro-immigration.

But Joe Biden’s feckless, reckless southern-border policy poisoned popular support for immigration. So now we have the Trump crackdown, and net immigration to America is plummeting.

Immigration numbers are the least accurate of our vital statistics, typically only tallied as a rear-view-mirror residual, after we get our figures for births, deaths and total population change.

But there’s no doubt 2025 is going to see very little net migration — if any.

In fact, a widely quoted new study from the American Enterprise Institute (where I work) suggests 2025 could be a year of net emigration from America, with half a million more foreigners leaving than arriving.

That is the study’s “outer boundary” low-side estimate — but if it comes to pass, as some observers have already noted, America’s population could actually decline very slightly this year.

This would be a first in US history.

Not even during the bloody Civil War or the deadly 1918 Spanish influenza did America’s numbers drop. They grew during the COVID pandemic, too.

The chances of US population decline this year still look like the longest of long shots: an outcome plausible, though highly improbable.

But Americans must realize that future population growth, if it occurs at all, will increasingly depend on immigration.

Depopulation, for its part, will be a “stress test” for our society, economy and political system — and it is by no means clear we are ready to pass that test yet.

Without immigration, America’s working-age population would shrink immediately — as in this year.

There’s a new, post-COVID drop in labor participation by older (55+) Americans, the population’s fastest-growing segment, and a continuing flight by prime-age (25-54) men.

Health trends are worrisome for non-Hispanic whites and blacks — about three-quarters of our population — and our byzantine health-care system, for all its advantages, makes treatment for their afflictions hellishly expensive.

No less problematic: Our 21st-century approach to public finance resorts to funding our immense entitlement programs through government borrowing — also known as future taxes on today’s children and even the unborn.

Government debt already exceeds gross domestic product and is set to balloon further in the years ahead under the Big Beautiful Bill.

In a depopulating America, the number of seniors will continue to climb (at least for a generation) while the working-age population supporting them dwindles — mismatching trends all but certain to hasten the welfare state’s crisis.

The golden age of postwar growth, it is true, came when immigration played little role — 1950 to 1973.

But that was a very different America: Family breakdown was in its early phases; men’s flight from work had not really gotten underway; Lyndon Johnson’s and Richard Nixon’s entitlement programs were only just revving up; and both political parties hadn’t abandoned budget discipline.

America had also reaped an invaluable dividend of talent from a small but extraordinary cadre of European refugees as a consequence of World War II — from Albert Einstein to Wernher von Braun, these minds contributed to our immediate postwar florescence.

To be clear: Shrinking and aging societies can prosper if they augment human resources under a business climate that helps unlock human beings’ value.

But America may actually be less prepared for depopulation today than it was a generation ago, given the array of bad national habits we have accustomed ourselves to recently.

Nicholas Eberstadt, the Wendt Chair in Political Economy at the American Enterprise Institute, is author of the forthcoming “America’s Human Arithmetic.”