JACOBY: These two groups were so similar that they couldn’t be told apart aside from this linguistic test of pronouncing that word and seeing who would pronounce the first “h.” It’s a recurring story in these fratricidal struggles, because brothers are virtually indistinguishable, and who can tell who’s who? You need this marker or indicator. Language is one. But then if it’s religion, you need other questions or tests. It’s [not that] there’s no differences, but they approach nothing at all. This is highly obvious with sibboleth-shibboleth, and killing someone for the one and not the other.

IDEAS: One of the more fascinating examples you give of minor difference is the “shibboleth” example, where in biblical times, the Gileadites would test the Ephraimites to pronounce the word correctly as the only way to tell their enemy from their friend.

JACOBY: Somalia is in some sense the perfect case. It’s one of the most homogeneous of all the African nations, and also one of the most riven. So what does homogeneity mean, and why do the smallest differences become the most acrimonious? This long-held idea of brotherhood seems questionable in overcoming large differences when the small ones are so divisive.

IDEAS: In your book, you say that while prior to the 20th century most civil wars could be tied to religious or political ideology, that has since become less and less true. How so?

Jacoby’s background is as an intellectual and cultural historian, but with this book, he has waded into subjects that are more often the domain of political historians, drawn by what he sees as a “kind of fetish” within academia to characterize conflicts as clashes with “the other.” Jacoby recontextualizes World War I as a continental-scale civil war; he points out that we might see the Holocaust not as the culmination of centuries of anti-Semitism, but rather as a backlash to hundreds of years of Jewish assimilation. Today, he points out, Islamic extremism can also be understood as a direct reaction to the flattening, assimilative power of globalization.

In a new book-length essay, “Bloodlust,” UCLA history professor Russell Jacoby begins with that first biblical murder. Perhaps surprisingly, he argues that violence has been and remains largely fraternal and unideological, and that it frequently hinges more on similarities than on difference. In fact, it is that very commonality that can make fraternal violence so intense — take our own Civil War.

Yet what of the conflicts that arise between neighbors or relatives? Are these the more common enmities? Are they, in fact, more brutal? The first fight in Judeo-Christian heritage, after all, is between siblings: “Cain rose up against Abel his brother, and slew him.”

Why do we fight? Or, to put it another way, whom do we fight? Often we assume that it’s invading hordes or usurpers from alien cultures. And sure, cultures clash, empires colonize. Our conflicts with Iraq and Afghanistan, many would argue, arise from the grand differences between the societies and customs of the West and of the Middle East.

IDEAS: And throughout this history, people also just give up on trying to tell the difference and slaughter everyone.

JACOBY: Well, yes. This goes back to the Albigensian Crusade around 1200 in France, where it was declared, “Kill them all. The Lord knows his own.”

IDEAS: Not many people are going to accept that all the players in World War I, for example, were homogeneous.

JACOBY: The question is, from an anthropological viewpoint, to what extent do these foreigners who were attacking each other — the Germans, the French, the English — differ from one another?

IDEAS: As you point out, several of the major leaders of those countries were blood relatives.

JACOBY: Right, and they killed each other with incredible brutality and abandon and dispatch. And so I see it as a European civil war. Not only, but basically. So that involves some rethinking and refiguring. These aren’t strangers, but they begin to become “others” to each other, to demonize each other — especially the Allies did this to the Germans. Yesterday’s little differences become today’s huge ones.

IDEAS: To move ahead to World War II, you quote a Vichy French official who calls Jews the “unassimilable foreigner,” and yet throughout the continent Jews often predated their neighbors.

JACOBY: Yes, it’s virtually the opposite. They were only thought of as unassimilable because they were so assimilated you could no longer tell who they were. The idea was, “If they start wearing these stars, then we’ll know.” Some of these anti-Semites lamented that their Jews weren’t like the eastern Jews, who dressed a certain way and were readily recognizable.

IDEAS: Ultimately you get into our current conflict, often portrayed as a cultural war with Islam, which has itself been categorized as unassimilable.

JACOBY: I am pursuing a logic...about how it is similarities that become the threat to Islamic fundamentalists, say. We see perhaps that the anger of Islamists in parts of the world is the fear that they’re losing their identity against modernity, or western consumerism, and that the small differences, as things contract, are more frustrated and endangered.

IDEAS: What would you say the book advocates?

JACOBY: There is no policy recommendation here. However, there’s a certain kind of psychological awareness that violence and hatred is familiar, and often familial, that is worth taking heed of. We can’t simply project it or look at it as an invasive force. It requires self-examination and self-consciousness.

IDEAS: How do you think about the recent disturbance in Afghanistan over the Koran burning?

JACOBY: It’s fascinating to me to what extent people are willing to risk their lives for these things. We’re talking about people whose countries are suffering incredible political and social collapse, and what brings out their ultimate murderous rage? Some crazed pastor burning a Koran in Florida. How can one explain that? It’s [that they’re] feeling so threatened in this world.

IDEAS: And what about in this country, in regard to our relationship to, say, Mexico?

JACOBY: Take the English-only movement. The fact is that Latinos in America assimilate at an incredibly rapid rate. They give up their language in a very short amount of time compared to other ethnic groups, and yet this English-only issue targets them as though they are a threat to encoded American ideals...from the American side, they fasten on tiny differences. It may as well be a fear of too many taco shops, or whatever. That seems like a grave threat to some people, but really it’s just another sign of assimilation.

J Gabriel Boylan is an assistant editor of Harper’s Magazine.

© Copyright 2011 Globe Newspaper Company.