In the end, Chua falls back on the very attitude to which she turned her sharply skeptical gaze at the beginning of the book: the conviction that the United States was, is and will remain an exceptional nation, different from all the others. In her introduction, Chua remarked that the United States as a supertribal entity indifferent to ethnicity and culture became at best a partial reality only a generation ago. By book’s end, however, the battered ideal has been polished and refurbished.

“With every wave of immigration in the past, American freedom and openness have triumphed. Will we, telling ourselves ‘These immigrants are different,’ be the weak link, the first generation to fail? Will we forget who we are?” That’s inspiring, and even more so are the citations of Martin Luther King Jr., Lin-Manuel Miranda and Langston Hughes that finish the book.

Inspiring — but not wholly reassuring. A lot of the interest of “Political Tribes” comes from the strong sense it emanates of an author arguing with herself. Chua both condemns tribalism and respects its power. She insists that the United States alone of nations among the earth has often transcended it — and then presents impressive contrary evidence from the past and the present. Chua reckons with the many tribalisms of the American past: ethnic, religious and racial. She hopes for a future in which tribalism fades — even as she mercilessly details its accumulating strength.

As Chua notes, tribes can coalesce out of previously unrelated pieces. Immigrants to Europe from North Africa, the Middle East and South Asia — of different languages, ethnicities, folkways and religious practices — raised children who created for themselves a new identity as post-ethnic Muslims. ISIS, as Chua mordantly observes, is in its own way a melting pot, bringing together young men and women from across Europe, Asia and the Americas to fight for a new ideology. She quotes a New York Times article about young British girls lured to ISIS. For them, “Islam is punk rock.”

Tribes can be created by fission as well as by fusion. Chua suggests that in the United States, divisions that would once have been understood as class divides have been reinterpreted in our time as cultural, even when they are not ethnic. “White Americans often hold their biggest disdain for other white Americans — the ones on the opposite side of the cultural divide.”

These fast-evolving and ever-changing identities may look contingent from the outside. They feel overwhelmingly powerful to those inside. Chua repeatedly scolds American policymakers for underestimating the importance of ethnocultural identity in Vietnam in the 1960s, Venezuela in the 1990s and Afghanistan in the 2000s. Through her book pulses an evident worry that tribal claims are now overpowering national ones within the United States. If she cannot quite bring herself to make her own anxieties explicit — or figure out what if anything to do to address them — she is hardly alone. As the rise of Donald Trump over more conventional politicians has so emphatically proved: Worsening social divisions are much easier to exploit than to explain or redress.