Finding a kind word for Genghis Khan, the Mongol warlord who built towering stacks of the heads of his enemies’ children, is the sort of comically revisionist exercise that can nevertheless make old history fresh again.

For Genghis — and for all world-class empires, from Persia to the present — the unhelpfully kind word Amy Chua finds, in her new “Day of Empire: How Hyperpowers Rise to Global Dominance — And Why They Fall” (Doubleday, 432 pages, $29.95), is “tolerant.” Received tales of Genghis’s pitiless subjugation of the known world obscure what was, she says, a willingness to tolerate the practices of many local groups within his dominion — broadmindedness remarkable at the time, even if the bloodshed and brutality are all we remember today. Accept vassal status, Genghis said, and you can keep not only your indigenous ways and leaders, but your children’s heads, too.

Genghis’s notion of tolerance is not, precisely, our own, admits Ms. Chua, a professor at Yale Law School and the author of “World On Fire” (2002). But it improved markedly on the even more rapacious practices of the day, she says. Genghis did, after all, accept surrender from his opponents as eagerly as he rejected it from his generals, and he wasn’t too picky about the tribal or ethnic kinships of the subjects who entered his fold.

Moreover, tolerance helped elevate the Mongol Empire from a regionally influential force to what Ms. Chua calls a “hyperpower.” The means by which the greatest empires have graduated from mere supremacy to the kind of full-court dominance displayed by the Mongols have been studied by ambitious statesmen for centuries, but Ms. Chua argues that “tolerance” has been a key overlooked ingredient. The Persians had it. So did Rome, the Tang dynasty, the Mongols, Spain before the Conquista, and many others, including America.

Ms. Chua sees tolerance as a useful binding agent that lets an empire take advantage of enemies and turn them to loyal servants, and that lets minorities within the empire occupy key niches in its structure, contributing talent and improving the functioning of the empire as a whole. Genghis’s vassals are an example of the first, and the Jewish bankers of early, modern Europe a flawless instance of the second. Her book describes the place of tolerance in each of more than a dozen hyperpowers and aspiring hyperpowers.

She cautions, though, that in the making of these hyperpowers’ greatness lay the “seeds of their decline.” (This type of tired phrasing, at its low points collapsing into cliché, is typical of the prose.) Tolerance breeds success and, ultimately, hyperpower status. But hyperpower status means taking in more territory and people than a liberal empire can control. In the end the tolerance breaks, and the hyperpowers diminish in fits of violent chauvinism.

This is a modest idea, and Ms. Chua spreads it thinly over several hundred pages of historical analysis. She has much to teach the lay reader about all the empires she examines, and particularly about the empires of Central Asia and the Far East. But her recitation of historical detail includes a great deal of elementary facts and figures (“President Kennedy was assassinated in 1963”) that are plainly out of place in a book of this sort, and that will be tiresome to all readers and downright tedious to those already broadly familiar with history.

And yet, the core ideas of “Day of Empire” are basically sound, though Ms. Chua presents them with eccentric and even misleading emphasis. She spends much of the book fending off the anticipated objections of those who would protest that the signal virtue of the British and Mongol empires was not “tolerance,” since like all empires they ruled by force and coercion. Her counterargument is to invoke the notion of “comparative tolerance”: that these empires, however illiberal by our standard, were more tolerant than any other empire or competitor at the time. By her lights, comparative tolerance was enough to dominate, since when it came to choosing sides between great powers, the one more likely to provide opportunities and advancement to aliens was the one to whom those aliens most eagerly submitted.

But the question arises: Is tolerance just a way to get enemies to cross the line and join one’s own side? Ms. Chua writes of tolerance in a sense that sounds unfamiliar to contemporary ears, because she treats tolerance as something useful, a sales pitch to the other team, a valuable trick that has worked well from Cyrus the Great to President Clinton. Indeed, in her examples we rarely see tolerance deployed except for reasons of self-interest, as the most efficient way to govern large territories with limited resources. What Ms. Chua means by “tolerance” is not the virtue of common usage, but a mercenary indifference to culture, to heritage, and to all that does not in some way feed the hungry, hungry hegemon.

The author mentions her own family’s history — a remarkable tale of ethnically Chinese immigrants from the Philippines who found laudable success in American academia and felt deep gratitude for the freedoms afforded them. She seems to suggest that in the spirit of toleration we keep these opportunities open for immigrants, especially industrious ones, and I find it difficult to disagree.

Since “tolerance” seems generally to be what Ms. Chua is urging on America now, we should have a clear-eyed awareness of what she means by the term, and what employing it has historically required from the other arms of empire. What really kept the empires running was a willingness to use extreme brutality alongside calculated conciliation. To single out toleration masks the savagery that made that toleration possible, and it overlooks the unsentimental practicality with which policies of toleration were instituted.

This elision leads to weird pronouncements not only about the Golden Horde but also about the Nazis, about whom Ms. Chua writes, “It is almost trivializing to call the Nazi regime ‘intolerant.'” Almost? A later paragraph states that by “murdering millions,” the Nazis “deprived themselves of incalculable manpower,” which instead fled the country. In such analysis, a little more sentimentality and a little less practicality would be greatly appreciated.

Mr. Wood is a staff editor at the Atlantic Monthly.