“Four score and seven years ago,” Lincoln said at Gettysburg on November 19, 1863, “our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal. Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived and so dedicated, can long endure.” The power and beauty of Lincoln’s speech have made those words among the most memorized and analyzed in American history. Yet as David Armitage points out in his lucid and learned book Civil Wars: A History in Ideas, we often gloss over a crucial phrase: “great civil war.” More than 150 years later, with the Civil War’s outcome undisputed, it is easy for us to take that term for granted. But at the time and for many decades after the conflict, it was referred to by a variety of names that each carried heavy interpretive loads: rebellion, revolution, war.

CIVIL WARS: A HISTORY IN IDEAS by David Armitage Knopf, 368 pp., $27.95

From the Union side, it could be seen as a rebellion by the South, subject to criminal prosecution under domestic law. Lincoln himself used “rebellion” far more often than “civil war,” and the official War Department history compiled in the late nineteenth century was called The War of the Rebellion. Meanwhile, from the Confederate side, it could be framed as a standard international war between different peoples, as the name “War Between the States” implied. As Armitage shows, describing the conflict as a “civil war” had important foreign and domestic implications, simultaneously affirming that both sides “remained members of the same political community” and highlighting the unity of the nation. The Union’s victory secured those meanings for the war, and they were further ratified when Congress finally settled on the official name of “Civil War” in 1907.

The questions the Civil War raised remain familiar to us today because they come up inevitably whenever we see internal conflicts within a country. Is the country one community or several? Is the conflict subject to domestic criminal law or the international laws of war? Could other countries legitimately intervene, and if so, using what justification? Such internal conflicts are becoming more and more common. By one measure, nearly 95 percent of all conflicts since the end of the Cold War have taken place within states rather than between them. As in places like Somalia, Rwanda, the Balkans, and now Syria, these wars often spill over their borders, either by drawing in other countries or sending out millions of refugees.

To cope with the seemingly intractable problem of civil war, lawyers have tried to fit it into the framework of international law, and social scientists have tried to define it precisely using data. Yet the ongoing confusion over Syria and other conflicts around the globe have eluded these attempts to impose rationality on ragged chaos. Armitage proposes a different approach: Can a history of ideas about civil war give us any guidance?

The term civil war originates in ancient Rome. The first recorded use—the Latin bellum civile—came in 66 BCE, when Cicero deployed it in a speech in the Forum. The way he used the term, without defining or explaining it, suggests the concept was familiar to his audience. Rome had experienced plenty of internal conflict between patricians and plebeians since its founding, but none rose to the level of war until 88 BCE. That year, the Roman general and consul Sulla opposed legislation introduced by the tribune Sulpicius, who wanted to extend the franchise. A riot broke out and Sulla withdrew from the city with his troops. He soon marched his army back to the city and took the Capitol. He declared his opponents public enemies and used dictatorial powers to undo Sulpicius’s laws. Then he sent his troops away and restored Rome’s usual governing structure.