There are stories Americans have long told themselves about their country, stories that — depending on the prevailing mix of cynicism, optimism and realism — still hold sway. The shining city on a hill. A nation of laws, not of men. The land of opportunity. The melting pot. With liberty and justice for all. Parsing the enduring truths and myths behind these notions (not to mention their original context) is the work of historians. Their choices of territory to explore and stories to interrogate are not just academic concerns, but reshape the mythologies on which nation-states are built, understood and mobilized.

In “This America,” a slim and pointed book, Harvard University historian Jill Lepore worries that her profession has ceased to tell the story of the American nation. For the past half-century, she argues, historians have veered toward sweeping global accounts, studying webs of commerce and culture, or to narrower investigations of class, gender and race. These approaches have produced vital scholarship from previously neglected vantage points, Lepore acknowledges, but bypassing the broader story of the United States carries risks. “Nations, to make sense of themselves, need some kind of agreed-upon past,” Lepore writes. “They can get it from scholars or they can get it from demagogues, but get it they will.”

Of course, sometimes scholars get demagogic, too. Lepore recalls how American and European intellectuals in the early 20th century “propped up the worst forms of national prejudice and national mythmaking,” with catastrophic results. Later, postwar historians sought to minimize internal dissent in the battle against communism, depicting ideological consensus and unchallenged Western liberalism, while coming late to the turmoil of civil rights. And starting in the 1970s, historians, fearing that nation-state histories would revive the scourge of nationalism, began to avoid them. It didn’t work. Historians left the door open to “fiends and frauds,” Lepore writes, those who promote hatred of outsiders rather than love of country.

It is no secret what sort of fiends and frauds she has in mind. Lepore decries the rise of Vladi­mir Putin in Russia, Viktor Orban in Hungary and Rodrigo Duterte in the Philippines, among others, as well as the Brexit vote in Britain — all signs of a nationalist resurgence. She reserves special ire for President Trump, “a nationalist with a vengeance,” and recalls a 2018 rally when he gleefully urged supportersto embrace the term: “I’m a nationalist. . . Use that word. Use that word!”

Though Trump seems bent on personally dismantling many of our national mythologies, Lepore is also critical of the American left for not offering comprehensive answers to this illiberal turn, for failing to draw on America’s story to fight back. “Trump’s loudest critics answered Trump’s viciousness with their own viciousness,” she writes, “his abandonment of norms with their own abandonment, his unwillingness to speak to the whole of the country with their own parochialism, speaking to their own followers rather than to the nation.”