A quarter-century ago liberal historian Arthur Schlesinger, Jr. warned of multiculturalism’s dangers: “If separatist tendencies go on unchecked, the result can only be the fragmentation, resegregation, and tribalization of American life.” In The Disuniting of America Schlesinger urged Americans to understand themselves as sharing, in Martin Luther King Jr.’s words, a “single garment of destiny.” But he knew the fight against diversity would be long and difficult: “In the century darkly ahead civilization faces a critical question: What is it that holds a nation together?”

We inhabit that dark century today. Debates over national identity and immigration envelope the Western world. Citizens increasingly register discontent with changing national identities and demographics at the polls: Britain’s exit from the European Union, electoral turmoil throughout the Continent, and the rise of Donald Trump. Yet too often, politicians who dare question diversity are accused of bigotry and xenophobia.

One man who hasn’t shied from the debate is Trevor Phillips, a politician, broadcaster, and former chair of the U.K.’s Equality and Human Rights Commission. His provocative call to arms, Race and Faith: The Deafening Silence, owes a great deal in style and substance to The Disuniting of America. A Labourite, and child of Guianese immigrants, Phillips argues that by inciting interest groups towards competition, conflict, and polarization, multiculturalism guarantees chaos.

Like Schlesinger, Phillips fears that multiculturalism will have its most harmful effects on progressives’ clients and ideals:

The greatest losers from the loss of social solidarity will, in the end, be minorities, whose capacity to maintain even the traditions which do not challenge mainstream values would wither in a backlash against superdiversity…. [S]queamishness about addressing diversity and its discontents risks allowing our country to sleepwalk to a catastrophe that will set community against community, endorse sexist aggression, suppress freedom of expression, reverse hard-won civil liberties, and undermine the liberal democracy that has served this country so well for so long.

Phillips is not alone. Liberal academics and think tanks are slowly starting to make the case for assimilation (or, to use the more acceptable term, “integration”). Clearly, Europe’s unintegrated Muslims can be turned into radical Islamists. A recent paper for the U.S.-based National Bureau of Economic Research outlined the five countries sending the highest number of ISIS fighters to Mesopotamia: Finland, Ireland, Belgium, Sweden, and Austria, all of which have seen a spike in political violence and concomitant opposition to refugees and immigrants.

American liberals are also reconsidering assimilation. After the San Bernardino massacre, New York University professor Jonathan Zimmerman wrote in Politico:

[N]either [Obama] nor observers on either side of the political aisle asked how an American citizen could attend 12 years of school and four years of college here, and then decide to attack his homeland…. Perhaps the threat of homegrown terrorism can spark a needed revival of civic education.

The Brookings Institution’s Daniel L. Byman has written that improving Europe’s border security and vetting refugees was important, but the “bigger, longer-term issue…remains that of integration.”

The lack of integration of refugees and minority communities can lead to radicalization, offering the Islamic State and other groups a potential pool of shooters and suicide bombers. Moreover, it enables suspects from these communities to hide from law enforcement. The Paris attack suspect, Salah Abdeslam…was able to avoid authorities for months in his home neighborhood of Molenbeek, despite being subjected to a continent-wide manhunt.

But, as Phillips recognizes, fear of terrorists is not enough to deeply unite nations: “[E]ven those of us on the progressive wing of politics must now surely accept that in the conditions of today’s society, our reflex defence of…separate communities is actually undermining one of the most cherished of left-wing values—social solidarity.”

Concern for social solidarity bonds old-style conservatives and old-style liberals alike. Most conservatives cherish community. Though holding that individual rights are derived from a creator, they believe that the good life is based on concentric circles of family, community, town, and state. And liberals shouldn’t have trouble seeing that a fractured society makes attaining income redistribution and egalitarianism hard.

Even before Harvard’s Robert Putnam reluctantly informed us in 2007 that diversity was divisive—that it caused community members to turn inwards, withholding indispensable trust and thus lowering local human and social capital—social scientists were studying “ethnic fractionalization.” The field’s gold standard is a 2003 Journal of Economic Growth paper titled, simply, “Fractionalization.” Both ethnic and linguistic fractionalization, it found, are highly correlated with “negative outcomes in terms of the quality of government.” Ethnic fractionalization, the probability that two random individuals belong to two different groups, “is highly negatively correlated with GDP per capita growth, schooling and telephones per capita…financial depth, fiscal surplus.” For egalitarian liberals like Schlesinger and Phillips concerned with income redistribution, the authors find a negative association: “It seems that governments have a much more difficult task achieving consensus for redistribution to the needy in a fractionalized society.”

Religious pluralism, by contrast, conditions people to accept different ideas, fostering the tolerance required for multiparty democracy. Religious fractionalization, the article notes, “bears a positive relationship to controlling corruption, preventing bureaucratic delays, tax compliance, transfers, infrastructure quality, lower infant mortality, lower illiteracy, school attainment, democracy and political rights.” Unsurprisingly, the least religiously fractionalized countries are predominantly Muslim.

“Fractionalization,” it should be noted, is not the same as “polarization,” which increases as group members grow farther apart. While fractionalization leads to low social capital and bad governance, polarization begets civil war. As Jose Garcia-Montalvo and Marta Reynal-Querol showed in 2002, nine of the top ten most polarized countries in the world experienced civil war during their sample period, while only four of the top ten most ethnically fragmented countries resorted to armed conflict. “The identification felt by an individual increases with the number of people in the group,” they wrote. “On the other hand the alienation felt by an individual from others depends on how far away are the latter from the individual.” In other words, deepening ethnic identity within the groups and increasing the distance between groups foments polarization.

Deepening ethnic group identity has been Western policy for several years now, and what Schlesinger and Phillips rail against. It doesn’t matter whether the groups are real, like Hutus and Tutsis, or artificial, like U.S. “Hispanics.” “It does not matter for our purposes whether ethnic differences reflect physical attributes of groups (skin color, facial features) or long-lasting social conventions (language, marriage within the group, cultural norms) or simple social definition,” according to “Fractionalization.”

When people persistently identify with a particular group, they form potential interest groups that can be manipulated by political leaders, who often choose to mobilize some coalition of ethnic groups (‘us’) to the exclusion of others (‘them’). Politicians can mobilize support by singling out some groups for persecution, where hatred of the minority group is complementary to some policy the politician wishes to pursue.

For nearly two centuries, our assimilation policy allowed the U.S. to be both ethnically diverse and relatively unified. Not being a nation defined by blood and soil—like most in Europe—America required that its citizens adhere to shared values and habits. And, as a constitutional republic, it required its citizenry take civic responsibility seriously. The founders created a system that, through common and later public schools, made Americans of natives and immigrants alike.

Progressives on both sides of the Atlantic set to work dismantling that model in the 1960s, a process decried by Schlesinger and Phillips. Schlesinger cites the 1989 report by New York City’s “Task Force on Minorities: Equity and Excellence,” which claimed that minorities “have all been the victims of an intellectual and educational oppression that has characterized the culture and institutions of the United States and the European American world for centuries.” The dominance of “the European-American monocultural perspective” purportedly explained why “large numbers of children of non-European descent are not doing as well as expected.” The Constitution, according to the report, is “vulgar and revolting.” As Schlesinger notes, the report’s recommendations for school curriculum take “no interest in the problem of holding a diverse republic together. Its impact is rather to sanction and deepen racial tensions.” New York’s experiment was replicated nationwide.

Phillips faults post-60s British policy for mimicking America even though race relations were very different in the two countries. He recalls that it was Roy Jenkins, Labour’s Home Secretary in 1965, who affirmed that “integration” henceforth would not be like “the flattening process of assimilation, but as equal opportunity, accompanied by cultural diversity, in an atmosphere of mutual tolerance.”

There is an evident distaste among Western elites, Phillips notes, “for any serious debate about the consequences of our growing ethnocultural diversity.” But we must debate these questions to discover which attitudes and behaviors are compatible with constitutional republicanism, democracy, and assimilation—and which are not. Even apart from radical Islam’s threat, we must do this before our nation’s connective tissue breaks down.

As G.K. Chesterton saw, American civic pride is necessarily built on the Declaration of Independence, which “enunciates that all men are equal in their claim to justice, that governments exist to give them that justice, and that their authority is for that reason just.” There is enough here for agreement between conservatives and liberals who value freedom and justice. If they can put aside their political and philosophical rivalries, they can honestly debate multiculturalism and assimilation, and not let King’s “single garment of destiny” be shredded by diversity.