For the first time since the 1930s, the number of immigrants living in America is falling, the Pew Research Center reports. It’s a whiplash-inducing turn of events, coming as it does on the heels of the Biden administration, which saw the largest wave of migration — “an unprecedented surge,” the Times marveled — in American history. The implication is that President Trump is fulfilling a campaign vow to control the border. What’s next?

In June, Pew reports, America’s foreign-born population stood at 51.9 million. That was a drop of some 1.4 million from January, when Mr. Trump took office. The foreign-born share of the population dropped, too, to 15.4 percent, per Pew, a drop from the recent record high of 15.8 percent. The numbers are historic, in the Times’s telling, since America last “experienced negative net immigration in the 1930s, during the Great Depression.”

An outfit that favors limits on immigration, NumbersUSA, is touting the falling number of foreign-born as “a good thing” for working Americans “who will benefit from a tighter labor market,” as the group’s Jeremy Beck puts it. That points to the debate that has been sparked over immigration policy within Mr. Trump’s MAGA movement. The contention centers on whether to encourage more legal immigration to bolster the workforce.

When this debate first erupted over the use of so-called H-1B visas coveted by Silicon Valley tech firms, these columns saw the moment as a chance for Mr. Trump to “forge a compromise.” The contours of such an agreement, we suggested, would feature, on the one hand, cracking down on illegal immigration and taking control of the border, “while expanding levels of legal migration that America’s economy needs to thrive.”

The death in March of Senator Alan Simpson served as a reminder that this debate among conservatives has been going on for decades. In the early 1980s, Simpson was adamant about getting control of the southern border. We recounted his “epic feud with the editorial page of the Wall Street Journal,” which, we said, advocated “to oversimplify — for letting the market regulate immigration.” The debate reflected migration’s cultural and economic tensions.

Mr. Trump’s progress on immigration, though, promises to transcend some of the limitations of this debate. That’s because getting control of America’s borders can prevent many of the cultural disruptions caused by unchecked illegal immigration. That gives America the breathing room, as it were, to be more selective and foster migration that the economy needs — especially high-skill labor to help fuel the tech sector and other innovative industries.  

This offers a chance, too, to reignite the engines of assimilation, which have come under increasing criticism by the left. Feature, say, the English language as a common bond. Why not seize the chance to ramp up the requirement for English fluency as a condition of citizenship? Why not reappraise civil rights laws that require, say, election ballots in foreign languages? Such inducements of cultural cohesion could dovetail with an expansion of legal immigration. 

A revived Melting Pot could vindicate Ronald Reagan’s remarks in 1989 that “other countries may seek to compete with us,” but “as a beacon of freedom and opportunity that draws the people of the world, no country on Earth comes close.” Quoth Reagan: “You can go to live in Germany or Turkey or Japan, but you cannot become a German, a Turk, or a Japanese. But anyone, from any corner of the Earth, can come to live in America and become an American.”