But for those who believe that assimilation is a matter of identity — as many on the far right do — nothing short of the abandonment of all traces of your heritage will do. The alt-right pundit Milo Yiannopoulos, an immigrant himself, told a campus group in January that ‘‘the hijab is not something that should ever be seen on American women.’’ The perception that visible signs of religious identity are indicators of deep and sinister splits in society can lead to rabid fears of wholly imaginary threats. Several states have passed anti-Shariah measures, in fear that Muslims will seek to impose their own religious laws on unsuspecting Americans. The fact that Muslims make up 1 percent of the U.S. population and that such an agenda is both a statistical and a Constitutional impossibility has done nothing to temper this fear. It is no longer a fringe belief: The white nationalist Richard Spencer told a reporter that he once bonded with Stephen Miller, now a senior White House adviser, over concerns that immigrants from non-European countries were not assimilating.

Debates about assimilation are different from debates about undocumented immigration, even though they are often mixed together. Concerns about undocumented immigration typically center on competition for jobs or the use of public resources, but complaints about assimilation are mostly about identity — a nebulous mix of race, religion and language. In May, a survey by the Public Religion Research Institute and The Atlantic found that white working-class voters were 3.5 times more likely to support Donald Trump if they reported feeling ‘‘like a stranger in their own land.’’ My seatmate on that airplane was a small-business owner, yet he did not seem worried about Korean-Americans taking business away from him; he seemed more aggrieved that their children studied two languages, or that his community featured store signs and church marquees in an alphabet he could not read. Others might object to their neighbors’ wearing skullcaps, or eating fermented duck eggs, or listening to Tejano music — and call these concerns about assimilation, too.

It should be clear by now that assimilation is primarily about power. In Morocco, where I was born, I never heard members of Parliament express outrage that French immigrants — or ‘‘expats,’’ as they might call themselves — eat pork, drink wine or have extramarital sex, in plain contradiction of local norms. If they do adopt the country’s customs or speak its language, they aren’t said to have ‘‘assimilated’’ but to have ‘‘gone native.’’ In France, by contrast, politicians regularly lament that people descended from North African immigrants choose halal food options for school lunches or want to attend classes in head scarves. One result is a daily experience of rejection, which only makes assimilation more difficult.

America is different from Europe in one significant way: It has a long and successful history of integrating its immigrants, even if each new generation thinks that the challenges it faces are unique and unprecedented. It is a nation in which people will wear green on St. Patrick’s Day without thinking much about the periods during which the Irish were accused of contaminating the nation with their foreign habits. Because there is no objective measure of assimilation, many people end up throwing up their hands and saying, ‘‘I know it when I see it.’’ The question is: Who is doing the judging here?