President Donald Trump has ordered 5,200 troops to the U.S.-Mexico border to repel what he calls an “invasion.” But the ongoing rush of Central American migration is significantly smaller than earlier surges. And like the previous wave of migration from Mexico, it’s probably not going to last. There simply aren’t enough people in Central America.

“There’s just not much demographic potential for mass migration from Central America,” said Douglas Massey, who has spent the last three decades tracking migration as a co-founder of the Mexico Migration Project. “It is heading in the same direction as Mexico — toward an aging population with limited growth, only the base population is much smaller.”

Reuters People walk in a caravan of migrants departing from El Salvador en route to the United States, in San Salvador, El Salvador, Oct. 31, 2018.

By the numbers, there is nothing resembling a “national emergency” on the border, as Trump has called it. Border Patrol arrested fewer than 400,000 migrants last year. For comparison, that figure for the year 2000 was 1.6 million — back when Border Patrol had about half the number of agents it now employs. Arrest rates for the last eight years have hovered at similarly low levels not seen since the early 1970s.

The reason arrests are so low is that mass Mexican migration to the U.S. ended a decade ago and shows no sign of resuming. Despite Trump’s fury, he took office under conditions more favorable to immigration hard-liners than any president since Richard Nixon.

Mexican mass migration ended as the fertility rate dropped

Neither Border Patrol nor Immigration and Customs Enforcement can take as much credit for that as they might like. The U.S. financial crisis that began at the end of 2007 triggered the drop in unauthorized Mexican migration by reducing labor demand. But perhaps more importantly, the crisis hit as the Mexican baby-boom era was petering out. In 1970, the fertility rate in Mexico stood at nearly seven children per woman. Today, it’s around 2.2, according to the World Bank.

The birthrate matters, because most people who migrate to the United States without authorization do so before they turn 30. As Mexico’s population has aged, the number of migrants attempting to cross into the United States illegally has fallen.

The U.S. has seen large-scale migration from Central America for decades, largely as a result of mass violence created by Cold War-era civil wars and authoritarian regimes that the U.S. government played a key role in funding and manipulating.

The defining change for unauthorized crossings over the past few years is that the number of Central American families and unaccompanied children jumped by tens of thousands in 2014 and has remained high. This year marked a record for Border Patrol arrests of Central American families, the vast majority of which come from the so-called Northern Triangle countries of El Salvador, Guatemala and Honduras.

But Central America is headed down the same demographic path as Mexico. Together, the populations of those three countries total fewer than 31 million. That’s about a third of Mexico’s population in the year 2000, when border arrests peaked.

And those three countries have similarly low birthrates. Guatemala’s is the highest, at nearly three births per woman, according to the World Bank. For Honduras, that figure is 2.45 per woman, and for El Salvador it’s just under 2.1 — a figure barely touching the level that would replace the country’s population.

Mathematically speaking, that makes it all but impossible for the Northern Triangle to produce the high levels of sustained migration that the United States saw from Mexico for decades.

“It’s hard to think that this is going to be a major stream for a long time,” Nestor Rodriguez, a sociologist at the University of Texas in Austin who researches Central American migration, told HuffPost. “The demographics are not there.”

Immigration from El Salvador may have already peaked...

That phenomenon might already be underway in El Salvador. Border Patrol arrested more than 27,000 Salvadoran families in 2016, along with 17,500 unaccompanied children. This year, the number of families dropped by half, and the number of kids fell by nearly three-quarters.

Despite Trump’s fury, he took office under conditions more favorable to immigration hard-liners than any president since Richard Nixon.

That trend might be partly explained by a decline in violence. El Salvador has one of the world’s highest homicide rates, which is one of the factors that prompted people to migrate to the United States in higher than normal numbers from 2014 to 2016. Over the last year, though, the homicide rate dropped.

But the underlying reality is that El Salvador is a tiny country with a fertility rate below replacement level, and it has already lost some 1.4 million migrants to the U.S. since 1970 — about a fifth of its total population.

“They’ve peaked,” Rodriguez said of El Salvador. “Unless there’s a huge rise in violence or civil war, we’re not going to see any major upsurge.”

...and Guatemala and Honduras could be next

The two other Northern Triangle countries have more capacity to sustain higher levels of migration for longer periods of time. Honduras has suffered ongoing political instability since a 2009 coup, along with high levels of violence and poverty. Guatemala has a larger population, a higher fertility rate and large numbers of indigenous farmers struggling with crop failures — a trend some experts believe is linked to climate change.

But those countries still face the same constraints as El Salvador. They’re small. Their populations are aging. The number of migrants they send may rise or fall from one year to the next, but over the long term it will likely peak and decline. And it will probably happen for demographic reasons that U.S. policymakers don’t control.

Even if the uptick in Central American migration that began in 2014 continues for years on end — which it almost certainly won’t — these numbers remain small by historical standards.

“What if they were allowed to stay?” Duke University political scientist Sarah Bermeo wrote in an opinion piece over the summer for the Durham Herald-Sun. “In 2016, the U.S. government detained 224,854 people from El Salvador, Guatemala and Honduras — less than one-tenth of 1 percent of the U.S. population. Even if the rate was maintained for a decade, it would still be a much smaller share of the U.S. population than previous waves of migrants such as Irish, Italians and Russian Jews.”