If you live in a developed country, you are among those enjoying the “Long Peace,” a period marked by the absence of large scale interstate war since the end of 1945. It is the longest period of such calm in modern history. During this same time period, however, the world’s pockets of conflict have moved away from the frontiers and turned, instead, inward. 

“The Long Peace stands under a dark shadow—the shadow of civil war,” writes Harvard historian and Weatherhead Center Faculty Associate David Armitage, whose new book, Civil Wars: A History in Ideas tracks the evolution of human understanding of civil war over two millennia.

“The 300 years between 1648 and 1945 constituted an era of war between states; the last sixty years appear to be an age of war within states,” he writes. Since 1989, the world has seen an average of twenty intrastate wars going on at any given time. Intrigued, Armitage sought out a definitive book on civil war, perhaps a counterpart to Hannah Arendt’s On Revolution from the 1960s. But he came up short. So, he embarked on the “less studied past” and decided to write a book himself. What he uncovered was 2000 years’ worth of philosophers, military leaders, lawyers, poets, and historians struggling mightily to understand exactly what is a civil war.

Somehow civil war evaded categorization. It was a calamity unto itself, unlike any other type of conflict. In the collective conscious it was thought to be the most heinous and vile kind of war.  Fighting an aggressive foreign power and defending your border can be seen as a noble goal, but waging a civil war is disgraceful, almost unspeakable, because it exposes a fissure inside a polity—a lack of control. 

“If war is said to be hell, then civil war is the worst possible form of that hell precisely because the enemies are intimate and familiar, and sometimes related,” says Armitage.

Civil wars often have common characteristics, he found. Wars within states tend to last four times longer than wars between them, and they tend to involve the world’s poorest countries.  And they recur. “The most likely legacy of a civil war is further civil war,” as economist Paul Collier puts it....