What immigration crisis? U.S. isn't being swamped. (Numbers.)

The eyes of the country are fixed upon the U.S.-Mexico border. Controversy over President Donald Trump's policy of separating the children from parents accused of illegal entry — a practice he didn't begin, but temporarily scaled up with a so-called zero-tolerance policy toward asylum-seekers — has caused a flood of outrage. Whether Trump's apparent reversal of that policy, and his return to Obama-era practices, will mollify critics remains to be seen.

But the larger issue of illegal immigration from the south remains unsolved. Trump and his advisers, particularly Stephen Miller, have portrayed illegal entry across the Mexican border as a mounting crisis, necessitating dramatic action. Nothing could be further from the truth; the problem has slowly been resolving itself, and will likely continue to do so.

First, net immigration of Mexicans, by far the largest group of both authorized and unauthorized immigrants during the past four decades, has ended. The Mexican-born population in the U.S. — including both those who came legally and those who came illegally — peaked in 2007 at about 12.75 million, and has since fallen by about 700,000:

The number of unauthorized Mexican immigrants has fallen even more, by 1.1 million. In other words, during the past decade, the U.S. has seen a large number of unauthorized Mexicans return home, and a modest number of Mexicans come in through legal channels, leading to a net decline.

Why did this happen? Despite some of its regions being mired in a horrifically violent drug war, Mexico's economy has grown robustly — the country's per capita gross domestic product, valued at purchasing power parity, is now about $19,500, higher than China's. Mexico's fertility rate has also fallen to 2.24 children per woman, just slightly more than the replacement rate of 2.1 — that means people need to stay home to take care of aging parents and take over family businesses, instead of going north to work.

Because of the end of mass Mexican migration, the illegal immigration Trump is upset about is coming almost entirely from Central America. Because of violence, political instability and unpromising economic prospects, an increasing number of people from the so-called Northern Triangle countries of El Salvador, Guatemala and Honduras have been making the perilous trek north through Mexico, seeking refuge and work in the U.S.

Central America, and the Northern Triangle in particular, has picked up where Mexico left off. As the Mexican-born population has fallen, the Central American-born population has risen by almost the exact same amount.

A majority of the recent Northern Triangle immigration has been of the illegal variety. It is this immigration Trump has been railing against, and repeatedly expressing concern about the threat posed by the Salvadoran gang MS-13.

In the early years of the decade, this was unquestionably a crisis — not because of the numbers involved, but because of how many children were being put in peril. Unaccompanied children flooded the border, prompting former President Barack Obama to initiate some of the child-detention policies that Trump later expanded. The number of these children being apprehended at the border has stabilized, though it remains high.

But there are reasons to think that this Central American mass migration, like its Mexican predecessor, will taper off soon. First of all, as in Mexico, total fertility rates in the Northern Triangle countries are dropping. All three countries have now made the transition to small families, with El Salvador actually below the replacement rate. That means that as the current generation grows up, there will be more pressure to stay in the country.

Also, the Northern Triangle countries have seen their economies grow considerably. All are still considerably poorer than Mexico's per-capita of $14,200 in 2008, when net immigration from Mexico halted. But both El Salvador and Guatemala have passed the $8,000 per person level, at which point higher GDP tends to reduce emigration. In other words, continued economic growth will make migration from El Salvador and Guatemala shrink from now on. Honduras still has a ways to go, but is making steady progress.

So there is every reason to believe that the Central American immigration wave has peaked, and will now start to decline, just like Mexico's did a decade earlier. Meanwhile, even now the number coming in is small compared to the number of Mexicans who came to the U.S. in the 1990s and 2000s.

Although the issue of children at the border presents a moral crisis, Central American immigration is not at a crisis level overall, nor likely to become so. Trump is spreading alarm over a phenomenon that will probably dwindle away from now on. The problem of unaccompanied children at the border is a problem, and one that Trump will hopefully handle better in the future. But the U.S. isn't in danger of being inundated by Central Americans.

Noah Feldman is a Bloomberg Opinion columnist. He is a professor of law at Harvard University and was a clerk to U.S. Supreme Court Justice David Souter. His books include "The Three Lives of James Madison: Genius, Partisan, President."